ence of embryology. In the following extract the author, after setting forth the arguments that have been adduced in favour of this theory, thus them. opposes * * * * "Let us now consult experience for materials in order to reply to this question. We find that in all cases where opportunity has been afforded of tracing, by direct observation, the origin of an organism, it has taken place by propagation; whilst, on the contrary, not a solitary unexceptionable observation of a spontaneous origin exists in the records of natural history. Analogy is, therefore, completely in favour of the view that propagation is the only manner in which existing organisms are engendered. The objections which have been urged against this view, and the arguments which have been adduced in favour of a spontaneous production of parasites, rest chiefly on the ground that in many cases the origin of these organisms, by means of propagation, is inexplica ble; and is, therefore, held to be impossible. But it is overlooked that the assumption of their spontaneous origin is in reality merely a formal explanation, which leaves us completely in the dark respecting the true reasons and conditions of their production. Moreover, many of these reasons have latterly become invalidated by the progress of knowledge, since not merely the possibility, but also the reality of their propagation to other organisms, and the inducing conditions, have been demonstrated in various parasites; and although in this respect at present much appears mysterious, yet the numerous experiences of latter years must raise a hope in every unbiassed observer, that the further advancement of knowledge will clear up the obscurity which at present envelops this province, and will estab lish the origin of all parasites by propagation, to the exclusion of spontaneous origin."-P. 424. It follows, from this view of the question, that parasites are never a true product of disease, and cannot therefore originate from degenerated particles of the body, depraved secretions, &c.: a position which does not, however, invalidate the commonly-received and firmly-established opinion, that certain morbid changes of the body favour the development of entozoa by furnishing those conditions under which alone these animals can exist. "Thus, for example, vegetable parasites (fungi) do not in general develop themselves upon mucous membranes, until, by morbid processes, a deposit of coagulated fibrin, which serves as a bed, has become prepared for them, and until this exudation has passed into a state of putrid decomposition. An abundant secretion of mucus favours the development of worms which have entered the intestinal canal from without. Some states of the organism, on the contrary, disqualify it as a habitation for parasites. Thus, most of the entozoa in the intestinal canal are expelled by increased peristaltic action; some fluids of the body, as bile, urine, gastric juice, and some medicines, prove deleterious, and indeed fatal, to some of them; inflammation, or at least suppuration, may injure, and even destroy them." -P. 425. A part of the mystery still involving this enquiry has been cleared away by the admirable researches of Steenstrup, which were noticed in a former number of this Journal (see Med.-Chir. Rev., July, 1846, p. 22). The leading fact therein demonstrated, that there are, namely, in many classes, and especially among the entozoa, animals which act as nurses, giving birth to new beings without being truly prolific, elucidates many points in the reproductive process of parasites. The generation of the guineaworm (filaria medinensis) is so remarkable as to have given rise to some 1847.] On Malformations. 451 doubt as to its real character: "they are viviparous, and contain in their interior such a prodigious quantity of young that some maintain the worm is not an animal at all, but a membranous sheath filled with small worms." It is further peculiar, although the existence of a male is known, that as yet only female filaria have been found in the human body. It may then be asked-do females alone, while still young but after their impregnation, enter the body, because they there find conditions favourable to their further development? (L.c., p. 457.) All these circumstances seem to indicate a resemblance to some of the stages described and depicted by Steenstrup; but it is proper to remark that the nematoidea have not presented to that careful observer any distinct evidence of alternate generations, so that further information is required upon this interesting question. Among the parasitic animals one of the most curious is the acarus folliculorum, belonging to the arachnida, discovered by Simon of Berlin and since described by other observers. It is sometimes spoken of as occurring so frequently that its presence can scarcely be regarded as abnormal. We believe, however, that there is some mistake in this statement: at all events it is no easy matter to procure specimens for the microscope, a circumstance difficult to reconcile with some of the accounts that have been published. According to the author, the a. folliculorum very frequently exists in the hair glands of the human subject, on the nose, upper lip, and the glands of the beard, being sometimes solitary, whilst at other times ten or more are found in a single gland. There is no subject on which the comprehensive and accurate researches of modern anatomy have thrown so much light, as that relating to malformations. The older observers, troubling themselves but little with the general laws of organization, rested satisfied, like their brethren of the geological school, with regarding all deviations from the ordinary course of things as lusus naturæ. Speaking of these vitia primæ conformationis, Bischoff, in his interesting sketch, says justly," for a long time they were rather objects of affright and aversion, of superstition and of curiosity, than of scientific investigation." It is, however, certain that by far the greater number of congenital malformations are susceptible of explanation according to the known principles of development and of nutrition; and, when closely scrutinized, notwithstanding the wide departure from the normal process they may seem to present, they fall into definite classes recognisable by the embryologist as belonging to certain phases in the development of the foetus. So much, indeed, is this the case, that the study of malformations and of developmental anatomy mutually illustrate each other; and it is not too much to affirm that the former, as a philosophical investigation, could never have been successfully cultivated without the aid and the elucidations of the latter. These considerations will enable us to appreciate the definition which Bischoff has adopted, as best expressing the true nature of these deviations from the normal process of growth. "A malformation is then that deviation of form, affecting either an organism or an organ, which is so intimately mixed up with the primitive mode of origin and of development, * Wagner's Handwörterbuch der Physiologie. Band I. p. 860. that it can only happen in the earliest period of the embryonic life, or at least before the term of its completion." The author has given a fuller explanation, but in the same sense, in the following words: "The peculiarity of these malformations, and their essential difference from ordinary morbid changes, are explained by the following considerations :—immediately after birth almost all the organs exist in a condition which, with slight modifications of form, they retain throughout life. All organs, indeed, grow until they are perfectly developed; but this growth is, for the most part, merely a simple augmentation of bulk. A few organs only, as the sexual apparatus and the thymus gland, undergo at a later period comparatively important modifications, either developing themselves more highly, or, on the other hand, disappearing. Indeed, in adults, the changes of the body are, in the normal state, almost solely confined to renewal of material (metamorphosis of tissues), whilst the form of the organs, with very trivial modifications, remains unaltered. The case is different with the embryo and fœtus. Here, as the laws of development teach us, the various parts and organs of the body are gradually developed from the simple stroma of the ovum. During foetal life we have, therefore, not merely nutrition, as afterwards, but also development; and whilst, after birth, pathological influences only affect existing structures, or, at most, give rise to the introduction of heterogeneous matters, previous to birth, morbid influences extend their operation even to the development, so that pathological structures are generated, which differ considerably from those occurring after birth."-P. 481. The preceding remarks apply, it must be remembered, only to derangements of form. There are certain other congenital defects which are dependant upon abnormal nutrition, and which, consequently, do not essentially differ from similar disturbances after birth; such as encysted tumours and other morbid products. The more limited instances of abnormal position and excess of parts, belong to distinct classes. The following classification, adopted by Dr. Vogel, is convenient for studying this department of pathological anatomy, if such it can be called. "1st Class. Malformations, in which certain parts of the normal body are entirely absent, or are too small.-Monstra deficientia. "2nd Class. Malformations produced by fusion or coalescence of organs. Coalitio partium-Symphysis. "3rd Class. Malformations, in which parts in the normal state united-as for instance, in the mesial line of the body-are separated from each other-Clefts, fissures. "4th Class. Malformations, in which normal openings are occluded—Atresia. "5th Class. Malformations of excess, or in which certain parts have attained a disproportionate size-Monstra abundantia. "6th Class. Malformations, in which one or many parts have an abnormal position-Situs mutatus. "7th Class. Malformations of the sexual organs-Hermaphroditism.”—P. 487. To these, the author also appends "diseases of the foetus, and abnormal states of its envelops." In a subject so comprehensive and at the same time so full of details as the one under consideration, it is impossible to do more than to offer a few observations relating to some of the most important and interesting points. One of the latter is connected with the causes and mode of origin of twin and triplet monsters, whose bodies are more or less extensively adherent or fused, as it were, together. According to some authorities they arise by a coalescence of two separate germs, which to us appears the more 1847.] Arrest of Development. 453 probable explanation; or they depend, according to others, on a furcation of a single germ. The author without attempting to decide positively on either of these views, inclines to the latter; and adduces the following as the chief arguments in its favour. "1. The organs that are united are always similar organs; head with head, thorax with thorax, &c.; a fact that can only be explained in a very forced manner by the assumption of a coalescence of two germs. "2. There is a complete transition from the cases where two almost perfect individuals are attached at only a circumscribed spot of the body, to those where one individual bears only some trivial supernumerary parts, or other malformation, as, for example, fissure of the skull; in short, to cases whose origin no one would ascribe to a coalescence of two germs. "3. Finally, it is totally incomprehensible, how, in the case of two separated germs or ova, of which each must have its own membranes, a union of two embryos can take place; and it is just as little to be comprehended how, in such a union, often more than the halves of the two systems can be so intimately fused together, as we sometimes find to be the case. These are the principal reasons which lead me to agree in the opinion, that all twin and triplet monsters, with the exception of the cases of fœtus in fatu, proceed from a simple germ, or ovum."-P. 509. The doctrine we have advocated, to the effect that the larger number of malformations are the result of certain disturbances in the formative process, usually termed "arrest of development," receives the most striking elucidation from congenital defects connected with the termination of the alimentary canal and genital organs. The clue to this part of the enquiry is to be found in three leading facts: 1. That the posterior extremity of the intestinal passage presents in the early stage of its development a cul-de-sac. 2. That there is subsequently, but still at an early epoch, in both sexes, a cloaca, or common connexion of the genito-urinary organs and the intestinal canal. 3. That there are in the two sexes corresponding organs-such as the testes and the ovaria-the penis and the clitoris-the scrotum and the labia majora; and that, in the early development of these parts, they are remarkably similar to each other, so that, till the commencement of the sixth week after conception, the sex cannot be distinguished. It has even been lately affirmed that there is in the male a representative of the uterus, consisting of a median sinus in the prostatic portion of the urethra. A careful examination of specimens contained in anatomical museums, enlightened by a thorough acquaintance with embryology, will enable the observer to comprehend the true nature of these malformations. Thus, the imperforate anus or the deficiency of the lower extremity of the rectum is evidently dependant on the persistence of the cul-de-sac of the primordial alimentary canal. The specimens in which the rectum communicates, in the male with the urethra and in the female with the vagina, or in either sex with the urinary bladder, is owing to the original cloacal formation remaining, instead of disappearing, as in the normal process, at about the 10th or 11th week, when the canalis uro-genitalis becomes separated from the rectum and anus. The cases in which the greatest difficulties have arisen, and relating to which the most false notions have prevailed even among medical practitioners, are those that are said to be examples of hermaphroditism. In former times no doubts seem to have been entertained as to the possibility of the essential organs of generation, by which are meant the testis and the ovarium, co-existing in the same individual. That this was the opinion of Hunter is made evident by his account of the Free Martin, or defective cow-calf. In his introductory observations he says, 66 as the distinction of male and female parts is natural to most animals; as the union of them in the same animal is also natural to many; and as the separation of them is only a circumstance making no essential difference in the structure of the parts themselves, it becomes no great effort or uncommon play in Nature, sometimes to unite them in those animals in which they are naturally separated, a circumstance we really find takes place in many animals of those orders in which such an union is unnatural." He subsequently expresses his belief that this unnatural hermaphroditism "now and then occurs in every tribe of animals having distinct sexes." Similar views have been prevalent up to a much later period; but it is apparent that, until each successive step in the evolution of the sexual organs in the embryo had been determined, it was not possible justly to interpret the character of the malformations under consideration. The researches of Baer, J. Müller, Rathke, Valentin, and other observers of the present day, which have thrown so much light upon the history of development, have at the same time removed the principal difficulties involved in hermaphroditical conformations, and have necessitated a review de novo of the whole question. One of the highest authorities in this branch of anatomical science, Professor Bischoff, and to whose masterly sketch we have already alluded, speaking of the cases formerly recorded, in which male organs were affirmed to exist on one side of the body, and female organs on the other; or in which male and female organs were supposed to be present on both sides in the same individual, judiciously observes, "there are so many probably explanations derived from the history of development as to how these appearances may have originated, partly owing to an arrest of formation, and, partly owing to modifications of individual types of development, that I cannot arrive at an unqualified decision." He afterwards expresses himself more definitely :-" if my view of the so-called hermaphroditical formations be correct, then, strictly speaking, there are none such in the higher animal forms and man; that is, there is no contemporaneous presence of testes and ovaria in one and the same individual: there are, respect being had to these essential organs, only male and female individuals.” We have already said that the complex and peculiar formations connected with the inferior part of the intestinal canal (representing the future rectum), and in an especial manner the complex productions and metamorphoses of the allantois within the body, and of the uro-genital canal, bear intimately upon the question under consideration. It is essential to know, for example, that in the early state of the embryo, the clitoris and the penis have a perfect resemblance to each other; that each possesses a particular groove; that there are in the male cutaneous folds corresponding to the labia majora of the female; that these folds are at first separated by a fissure; and that subsequently they unite to form the |