animals. In the determination of these general principles, or laws of morphology, it is necessary to combine the knowledge of the anatomy and development of animals with that of man.

PLAN OF ORGANIZATION.

Vertebrate type. The general plan of construction of the human body agrees closely with that which prevails in a certain number of animals, viz., mammals, birds, reptiles, amphibia, and fishes, and is known as the vertebrate type of organization. The main feature of that type, and that from which its name is derived, belongs to the internal skeleton, and consists in the existence of a median longitudinal column, which extends through the whole trunk, and is composed in the fully developed state of a series of bones termed vertebræ. This vertebral column is formed in the early embryo around a simple rod-like structure, the primitive skeletal axis, which is called the notochord, and which in most vertebrate animals disappears to a greater or less extent in the course of development. The more solid portions of the vertebræ immediately surrounding the notochord are known as the bodies or centra (figs. 2 and 3), and constitute a pillar around which the other parts are grouped with a certain regularity of structure. At one extremity of this pillar is situated the head, showing in almost all the animals formed upon this type a greater development of its constituent parts; and at the other the tail in which an opposite character or that of diminution prevails; while on the sides of the main part or trunk, there project, in relation with some of the vertebral elements, two pairs of symmetrical limbs.

The head and trunk contain the organs or viscera most important to life, such as the alimentary canal and the great central organs of the vascular and nervous systems, while the limbs, from which such principal organs are absent, are very variable and differ widely in the degree of their development among the various animals formed upon the vertebrate type. In man and the higher animals the trunk is divisible into neck, chest, abdomen, and pelvis.

The vertebrate form of skeleton is invariably accompanied by a determinate and conformable disposition of the other most important organs of the body, viz. :— firstly, the existence on the dorsal aspect of the vertebral axis of an elongated cavity or canal which contains the brain and spinal cord, or central organs of the nervous system; and secondly, the existence on the ventral aspect of the vertebral axis of a larger cavity, the visceral cavity, body cavity or calom, in which are contained the principal viscera connected with nutrition and reproduction, such as the alimentary canal, the heart and lungs, the great blood-vessels, and the urinary and generative organs.

The general disposition of the parts of the body and of the more important viscera in their relation to the vertebral axis are shown in the accompanying diagrams of the external form and longitudinal and transverse sections of the human embryo at an early period of its existence.

Segmentation of the body.-The vertebrate type of organisation in the repetition of similar structural elements in a longitudinal series, has a segmented character, especially in the axial portion of the body, and this segmentation affects more or less, not merely the skeletal parts of its structure, but also, to some extent, its other component organs.

A segmented plan of construction is by no means restricted to vertebrate animals, but exists in several other classes of the animal kingdom, as is most conspicuously seen in the Arthropoda, such as insects and crustacea, and in the Annelida or worms. These animals, however, although showing a serial repetition of parts of like structure, are not considered to belong to the vertebrate type of organization.

In the human embryo, as in that of all vertebrate animals, the segmentation is most marked in the muscular system, the nervous and osseous systems becoming for the most part correspondingly marked off: in the adult the osseous and nervous systems retain in great measure the segmentation which has thus been produced, although in the muscular system it has

Fig. 1.-DIAGRAM OF AN EARLY HUMAN EMBRYO. (Allen Thomson.)

8, 8, indications of the vertebral divisions along the line of the back; r, u, upper limb; t, f, lower limb; u, umbilical cord. In the cranial part the divisions of the brain are indicated, together with the eye, and au, the auditory vesicle; near 6, the visceral arches and clefts of the head, forming inter alia the rudiments of the upper and lower jaws.

Fig. 2.-SEMIDIAGRAMMATIC VIEW OF A LONGITUDINAL SECTION OF THE EMBRYO REPRESENTED IN FIGURE 1; SHOWING THE RELATIONS OF THE PRINCIPAL SYSTEMS AND ORGANS. (Allen Thomson.) 1, 2, 3, 4, 5, primary divisions of the brain in the cranial part of the neural canal; n, n, spinal cord in the vertebral part of the canal; 8, spinous process of one of the vertebræ ; ch, chorda dorsalis running through the axis of the vertebral centra; ch', the same extending into the base of the cranium : a, dorsal aorta; p, pharyngeal cavity; i, i, alimentary canal: h, ventricular part of the heart, with which the arterial bulb is seen joining the aorta by arches; b, visceral arches of head; l, liver; 2, Wolffian body; , urinary vesicle or allantois, joining the intestine in the cloaca, cl; u, u', umbilicus.

Fig. 3.-TRANSVERSE SECTION (DIAGRAMMATIC) OF THE TRUNK OF THE EMBRYO THROUGH THE UPPER LIMBS. (Allen Thomson.)

m, spinal cord; n, neural or dorsal arch, including bone, muscle, skin, roots of the nerves, &c.; ch, chorda dorsalis, surrounded by the vertebral body or centrum; v, ventral or visceral arch, or wall of the body; P, P, body cavity; i, alimentary canal; h, heart; 7, 7, the rudimentary limbs.

ABOVE. 3.

C, centrum; N, neural cavity; V, cavity of the chest, visceral cavity.

become greatly obscured. To the original segments in the embryo the terms protovertebræ, mesoblastic somites or myotomes have been applied; those segments or metameres which are traceable in the adult are often spoken of as vertebral segments. In the limbs, although there is strong reason for believing that they have originated as outgrowths of certain segments of the trunk, the repetition of such vertebral elements, and their primitive connection, are greatly obscured.

Homology. A certain agreement in structure, situation and connection of parts or organs constitutes what is called homology, and this term is generally employed to indicate the morphological identity of representative parts in different animals, which may be considered to have its cause in community of origin (homogeny, Lankester), while the anatomical correspondence of parts which are repeated in the same animal may be more exactly distinguished as serial homology (homodynamy, Gegenbaur). Thus the arm-bone or humerus of a man is homologous (homogenetic) with the upper bone of the fore limb of a quadruped, or of the wing of a bird, while it is at the same time serially homologous (homodynamic) with the thigh bone of man himself, or any other vertebrate animal. It has farther been found convenient to express by the word analogy that kind of resemblance among the organs of animals which depends upon similarity of function, and although it may be accompanied by considerable agreement in structure, yet is not rendered complete by anatomical relation and connection: for example, the gills of a fish, of a crab, and of a mussel, serving the same function, are analogous organs, but in no sense homologous, as all morphological correspondence, or genetic relation, is wanting between them. Thus also, the upper limb of a man, the fore limb of a quadruped, the wing of a bird, and the pectoral fin of a fish are homologous but not analogous structures, the wing of a bat and the wing of a bird are both homologous and analogous, while the last is analogous to but not homologous with the wing of an insect.

Symmetry of form.-A remarkable regularity of form pervades the organization of certain parts of the body, especially the whole of the limbs, the head and neck, and the framework, at least, and external walls of the trunk of the body. Thus, if we conceive the body to be divided equally by a plane which passes from its dorsal to its ventral aspect (median plane), the two halves, in so far as regards the parts previously mentioned, correspond almost exactly with each other, excepting by their lateral transposition,-and the human body thus shows in a marked manner the character of bilateral symmetry. There is, however, a departure from this symmetrical form in the developed condition of certain of the internal organs, such as the alimentary canal from the stomach downwards, the heart and first part of the great blood-vessels, the liver, spleen, and some other viscera.

Descriptive terms. In the description of parts so numerous, so various in form, and so complex in their connections as those composing the human body, there is difficulty in finding terms which shall indicate with sufficient precision their actual position and their relation to the rest of the organism. This difficulty is farther increased by the exceptional erect attitude in which the trunk of the human. body is placed as compared with the horizontal position in animals. Hence, a number of terms have long been in use in human anatomy which are understood in a technical or restricted sense. For example, the median plane, already referred to, being that by which the body might be divided into right and left lateral halves, and the middle or median line being that in which the median plane meets the surface of the body, the words internal and external are used to denote relative nearness to and distance from this plane on either side, and may be replaced by mesial and lateral. The terms sagittal, frontal, and coronal, are also used in indication of direction within the body: sagittal denoting a dorso-ventral direction in or parallel to the median plane, frontal or coronal a transverse direction perpendicular to that

plane. The words anterior and posterior, superior and inferior, and several others indicating position, are employed in human anatomy strictly with reference to the erect posture of the body. But now that the more extended study of comparative anatomy and embryonic development is largely applied to the elucidation of the human structure, it is very desirable that descriptive terms should be sought which may without ambiguity indicate position and relation in the organism at once in man and animals. Such terms as dorsal and ventral, neural and visceral, cephalic and caudal, central and peripheral, proximal and distal, axial and appendicular, preaxial and postaxial, are of this kind, and ought, whenever this may be done consistently with sufficient clearness of description, to take the place of those which are only applicable to the peculiar attitude of the human body, so as to bring the language of human and comparative anatomy as much as possible into conformity. In many instances, also, precision may be obtained by reference to certain fixed relations of parts, such as the vertebral and sternal aspects, the radial and ulnar, and the tibial and fibular borders, the flexor and extensor surfaces of the limbs, and similarly in other parts of the body.

EMBRYOLOGY.'

By E. A. SCHÄFER.

FORMATION OF THE BLASTODERM.

STRUCTURE OF THE OVUM AND CHANGES PRIOR TO SEGMENTATION.

THE human body with all its tissues and organs is the product of the development of a single nucleated cell, the egg-cell, germ-cell, or ovum, which is formed within the principal reproductive organ of the female or ovary. The commencement of development is preceded by certain changes in the ovum, which usually occur soon after its discharge from the ovary, and consist (1) in the emission of certain constituents of the nucleus which form the so-called polar globules; (2) in the accession of the nucleus of a sperm-cell or spermatozoon, which is formed within the reproductive organ of the male (testicle), and which, blending with the remaining part of the nucleus of the ovum, appears to take the place of the part which was discharged in the form of the polar globules.

An account of the structure of the ovum, and of the manner in which the above changes are effected, may therefore appropriately precede the description of the actual course of development of the ovum.

Structure of the ovarian ovum.-The human ovum resembles that of all other mammals (with the exception of monotremes) in its minute size. Immediately before the time of its discharge from the Graafian follicle of the ovary in which it has been formed, it is a small spherical vesicle measuring about th inch (2 mm.) in diameter, and is just visible as a clear speck to the naked eye. When it is examined with the microscope, it is found to be invested by a comparatively thick, clear covering. This, when the centre of the ovum is exactly focussed, has the appearance in optical section of a clear girdle or zone encircling the ovum (fig. 5), and was hence named zona pellucida by von Baer (1827). But on more careful examination with higher magnifying powers, and especially by the examination of sections, there is not much difficulty in making out the existence of striæ passing radially through the membrane (fig. 6, zp). On this account, and especially since a similar radially striated membrane forms a characteristic part of the investment of the ovum in many animals belonging to widely different classes, it is more convenient, in place of the name zona pellucida, which has been exclusively used to designate this investment in mammals, to employ the more general term zona radiata, or to speak of it simply as the striated membrane of the ovum.

The zona radiata of the mammalian ovum is sufficiently tough to prevent the escape of the contents of the ovum, even when subjected to a considerable amount of pressure. If, however, the pressure be excessive, the tunic splits, and the soft

1 It is mainly owing to the researches of His, published principally in the important monograph "Anatomie menschlicher Embryonen" (Leipzig, 1880-1885), that our knowledge of the development of the human embryo is now far more complete than was the case when the last edition of this work was undertaken, and we are therefore able to keep more closely than was before possible to the human species in following the course of development of the ovum. For the elucidation, however, of many of the details of development, especially in its earlier stages, it will still be necessary to refer continually to facts which have been made out only from the study of the embryology of other mammals, as well as birds, reptiles, fishes, and even invertebrata.