of this with ectoderm at the margin of the blastopore, and in the higher forms, especially mammals, may even be largely derived from ectoderm. It is nevertheless for many reasons probable that the origin in a pair of hollow diverticula, as above described, is to be looked upon as the typical one, and that as a solid outgrowth, subsequently becoming split or hollow, as a secondary modification. It is questionable, however, whether there is so considerable a difference between the external and internal portions of the wall of the diverticulum that

Fig. 29.--SECTIONS ACROSS AN AMPHIOX US EMBRYO OF ABOUT THE STAGES SHOWN IN FIG. 28, I. To III. (Hatschek.)

n.g., neural groove; n.c., neural canal; ch, rudiment of notochord; mes. som., mesoblastic somite. In I., its cavity is in free communication with the alimentary cavity; ep, epiblast; hy, hypoblast; al, alimentary cavity. In III. the cavity of the somite has extended on either side of the alimentary canal and forms a coelom, or body cavity (ca).

these two plates of mesoblast should be regarded each one as of equal morphological importance with the epi- and hypoblast.

It may also be doubted whether the parablastic cr mesenchyme elements are essentially different in their origin from the rest of the mesoblast. In forms which are regarded as most typical, such as Sagitta and Amphioxus, they are not distinct in origin from that layer. In the simpler forms amongst the Craniata, as Cyclostomata and Amphibia, no origin distinct from the rest of the mesoblast has been described for these elements, nor has it been seen in mammals, in which, indeed, it is difficult to conceive an independent source for them. It is only in the highly modified meroblastic ova that appearances have been noted which have seemed to justify the ascribing a peripheral origin to the parablastic elements. But the evidence which has been hitherto adduced in favour of this view cannot be regarded as sufficient to justify its unconditional adoption, and it must be regarded as equally open to consideration whether the derivation from that part of the blastoderm which is most closely connected with the source of nutriment, viz., the yolk, of those elements which are to form the blood and blood-vessels, and otherwise to minister to the nutrition of the early embryo is not to be explained by the modified physiological conditions of these telolecithal ova.

RECENT LITERATURE.

Balfour, F. M., On the structure and homologies of the germinal layers of the embryo. Quart. Journ. Microsc. Sc., vol. xx., 1880.

Balfour, F. M. and F. Deighton, A renewed study of the germinal layers of the chick. Quart. Journ. Microsc. Scienc., vol. xxii., 1882.

Beneden, Ed. van, Recherches sur l'embryologie des mammifères. La formation des feuillets chez le lapin. Archiv. de biologie, t. i., 1880; Sur l'évolution de la ligne primitive, la formation de la notocorde et du canal cordal chez les mammifères (lapin et murin). Bulletin de l'académie royale de Belgique. Ann. v., sér. iii., t. xii., 1886.

Beneden, Ed. van und Ch. Julin, Observations sur la maturation, la fécondation et la segmentation de l'œuf chez les cheiroptères. Arch. de biol., t. i., 1880.

Bonnet, R., Ueber den Primitivstreifen und die Chorda der Wiederkauer. Sitzungsber. d. Gesellschaft f. Morphologie u. Physiologie zu München, 1886; Beiträge zur Embryologie der Wiederkäuer. Arch. für Anat., 1884, 1889.

1 R. and O. Hertwig, "Die Colomtheorie," Jena, 1881.

2 Cf. Balfour, "Comparative Embryology," vol. ii. pp. 296, 297.

Boveri, T., Ueber Differenzirung der Zellkerne während der Furchung des Eies von Ascaris megalocephala. Anat. Anzeig., 1887.

Bütschli, O., Beiträge zur Gastrulatheorie. Morpholog. Jahrbuch, Bd. ix., H. 3, 1884.

Caldwell, W. H., The embryology of Monotremata and Marsupialia. Philosophical Trans., vol. clxxviii., 1888.

Durham, H., Note on the presence of a neurenteric canal in Rana. Microscopical Science, N. Ser., vol. 26, 1886.

Duval, M., De la formation du blastoderme dans l'œuf d'oiseau. naturelles, 6 sér. t. xviii., No. 1-3, 1884.

Quarterly Journal of

Annales des sciences

Fleischmann, A., Zur Entwickelungsgeschichte der Raubtiere. Biolog. Centralbl., Bd. vii.,

1887.

Gasser, Der Parablast u. der Keimwall der Vogelkeimscheibe. Sitzungsber. d. naturw. Gesellsch. zu Marburg, 1883.

Gerlach, L., Ueber die entodermale Entstehung der Chorda. Biol. Centralbl., 1881. Giacomini, C., Sul canale neurenterico et sul canale anale nelle vesicole blastodermiche di coniglio. Giorn. della r. accademia di medic. di Torino, No. 4, 5, 1888.

Haddon, A., Note on the blastodermic vesicle of mammals. Proceedings of the Royal Dublin Society, N. Ser., vol. iv., 1885.

Haeckel, Die Gastraatheorie. Jena Zeitschrift, Bd. viii.; Nachträge zur Gastraatheorie. Ibid., Bd. xi.; Ursprung u. Entwickl. d. thierischen Geweben. Ibid., Bd. xi.

Hatschek, B., Studien über Entwicklung des Amphioxus. Arbeiten a. d. zool. Instit. zu Wien, Bd. iv., 1881; Ueber die Entwicklung des Amphioxus. Biolog. Centralbl., Bd. vi., 1887.

Heape, W., The development of the mole (Talpa europaea). The formation of the germinal layers, and early development of the medullary groove and notochord. Quart. Journ. of Microsc. Sc., N. S., xci., 1883. Also in Studies from the Morphological Laboratory in the University of Cambridge, vol. iii.

Hensen, Ueber die Ableitung der Umkehr der Keimblätter des Meerschweinchens. Verhandlungen des physiologischen Vereins in Kiel, 1881.

Hertwig, O., Die Entwicklung des mittleren Keimblattes der Wirbelthiere. Jena Zeitschrift für Naturw., Bd. xvi., 1882. Versuch einer Erklärung des mittleren Keimblattes.

Hertwig, O, und R., Die Cölomtheorie.

Jena, 1881; Studien zur Blättertheorie. Jena, 1883.

His, W., Der Keimwall des Hühnereies u. d. Entstehung der parablastischen Zellen. Zeitsch. f. Anat. u. Physiol., Anat. Abth., 1876; Die Lehre vom Bindesubstanzkeim (Parablast), Rückblick nebst critischer Besprechung einiger neuerer entwicklungsgeschichtlicher Arbeiten. Archiv für Anat. u. Physiol., Anatom. Abtheil., 1882.

Hoffmann, C. K., Die Bildung des Mesoderms, die Anlage der Chorda dorsalis und die Entwickelung des Canalis neurentericus bei Vogelembryonen. Amsterdam, 1883.

Hubrecht, A. A. W., Die erste Anlage des Hypoblastes bei den Säugethieren. Eine Erwiderung an Herrn Prof. Ed. van Beneden. Anatomischer Anzeiger., iii. Jahrg., No. 30, 1888. Johnson, Alice, On the fate of the blastopore and the presence of a primitive streak in the Newt (Triton cristatus). Quart. Journ. of Microsc. Science, N. S., No. xcvi., 1884. Keibel, F., Van Beneden's Blastoporus und die Rauber'sche Deckschicht. 1887; Die Entwickelungsvorgänge am hinteren Ende des Meerschweinchenembryos. Arch. f. Anat. u. Physiol., Anat. Abth., H. 5 u. 6, 1888.

Anatomisch. Anz.,

Koller, C., Untersuchungen über die Blätterbildung im Hühnerkeim. Arch. f. mikr. Anat., 1881; Beiträge zur Kenntniss des Hühnerkeims im Beginne der Bebrütung. Wiener Sitzungsber., Bd. lxxx., 1881.

Kölliker, Die Entwicklung der Keimblätter des Kaninchens. Würzburg Festschrift. Leipzig, 1882; Die embryonalen Keimblätter u. d. Gewebe. Zeitschr. f. wiss. Zool. xl., 1884; Ueber die Nichtexistenz eines embryonalen Bindegewebskeims (Parablasts). Sitzungsber. d. phys. medic. Ges. zu Würzburg, 1884.

Kollmann, J., Der Mesoblast und die Entwickelung der Gewebe bei Wirbelthieren. Biolog. Centralblatt, Bd. iii., 1884; Der Randwulst u. d. Ursprung d. Stützsubstanz. Arch. f. Anat. u. Physiol., Anat. Abth. 1884.

Kowalevsky, Entwicklungsgeschichte der Sagitta. St. Petersburg Memoirs, xvi., 1871; Entwicklungsgesch. d. Amphioxus, &c. Arch. f. mikr. Anat., Bd. xiii., 1877; Ueber die ersten Entwickelungsprocesse der Knochenfische. Zeitschr. f. wissenschaft. Zoologie, Bd. xliii., H. 3, 1886. Kupffer, C., Das Ei von Arvicola arvalis und die vermeintliche Umkehr der Keimblätter an demselben. Münchener Sitzungsberichte, H. 5, 1882; Die Gastrulation an den meroblastischen Eiern der Wirbelthiere und die Bedeutung des Primitivstreifs. Archiv f. Anat. und Physiol., Anat. Abtheil., 1882; Ue. d. Canalis neurentericus der Wirbelthiere. Sitzungsberichte der Gesellsch. f. Morphol. u. Physiologie zu München, 1887.

Lankester, On the primitive cell-layers of the embryo, &c. Annals and Mag. of Nat. Hist., xi., 1873; Notes on embryology and classification. Quarterly Journ. of Micr. Science, xvii., 1877. Mitsukuri, K., and Ishikawa, C. On the formation of the germinal layers in Chelonia. Journal of the College of Science, Imperial University, Japan, vol. i., 1888.

Morgan, Notes on the fate of the amphibian blastopore, John Hopkins University Circulars, 1889. Perényi, I., Die Entwickl. der Keimblätter u. d. Chorda in neuer Beleuchtung, Anat. Anzeiger, 4. Platt, Studies on the primitive axial segmentation of the chick, Bulletin of the Zool. Museum at Harvard College, 1889.

Rabl, C., Ueber die Bildung des Mesoderms. Anatomischer Anzeiger, iii. Jahrg., No. 23-25, 1888.

Rauber, Ueber d. Ursprung des Blutes u. der Bindesubstanzen. Sitzungsh. d. Naturf. Gesellsch. zu Leipzig, 1877; Die Entwickelung der Gewebe des Säugethierkörpers und die histologischen Systeme. Ber. der Naturf. Ges. zu Leipzig, 1883.

Ravn, E., Ueber die mesodermfreie Stelle in der Keimscheibe des Hühnerembryo. Archiv f. Anatomie u. Physiologie, Anatom. Abth., 1886.

Repiachoff, W., Bemerkungen über die Keimblätter der Wirbelthiere. Zool. Anzeiger, vi., 1883. Romiti, G., Sur l'origine du mésoderme et ses rapports avec le vitellus. Arch. ital. de biologie,

t. ii., 1882.

Ueber die künstliche Hervor

Roux, W., Beiträge zur Entwickelungsmechanik des Embryo. bringung halber Embryonen durch eine der beiden ersten Furchungskugeln, dc. Virchow's Archiv, Bd. cxiv., 1888.

Rückert, Ueber die Gastrulation der Selachier. Biologisches Centralblatt, Bd. vi.

Ryder, The inversion of the blastodermic layers in Hesperomys. American Naturalist,

vol. xxi.

Schanz, F., Das Schicksal des Blastoporus bei den Amphibien. Jenaische Zeitschr. f. Naturwissensch., Bd. xiv., 1887.

Selenka. Emil, Keimblätter und Primitivorgane der Maus. Wiesbaden, 1883; Die Blätterumkehrung im Ei der Nagethiere. Wiesbaden, 1884; Ueber die Gastrulation der Knochenfische und Amnioten. Biolog. Centralbl., 1887.

Shipley, A. E., On the formation of the mesoblast, and the persistence of the blastopore in the lamprey. Proceedings of the Royal Society, 1886; On some points in the Development of Petromyzon fluviatilis. Quarterly Journal of Micros. Science, No. 107, 1887.

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Spee, F., Beitrag zur Entwickelungsgeschichte der früheren Stadien des Meerschweinchens bis zur Vollendung der Keimblase. Arch. für Anat. u. Physiol., Anat. Abth., Heft. i. u. ii., 1883. Spencer, On the fate of the blastopore in Rana temporaria. Zool. Anzeiger, 1885.

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EARLY CHANGES IN THE BLASTODERM, RESULTING IN THE FORMATION OF THE EMBRYO.

FIRST APPEARANCE OF THE EMBRYO; FORMATION OF THE NEURAL GROOVE AND MESOBLASTIC SOMITES.

Neural canal.-The blastoderm in mammals, as we have seen, eventually completely encloses the cavity of the ovum, and even in the large telolecithal ovum of the bird, it gradually extends so as to cover a large part of the yolk. But only a small portion of this membrane takes part in the formation of the body of the embryo, that portion namely which lies immediately in front of the primitive groove, and it is here that what may be regarded as the first trace of the embryo makes its appearance-very soon after the outgrowth of mesoblast from the sides of that

Fig. 30.-EMBRYONIC AREA, WITH OUTLINE OF PART OF THE VASCULAR AREA, FROM A RABBIT'S OVUM OF SEVEN DAYS. 2. (From Kölliker.)

oo, vascular area; ag, embryonic area; pr, primitive streak and groove; rf, medullary groove.

groove-in the form of a shallow furrow (fig. 30, rf), wide behind where it embraces the anterior end of the primitive streak, and at first narrow in front, and bounded on either side and anteriorly by a fold of epiblast; which folds, in fact,

Fig. 31.-SECTION ACROSS THE ANTERIOR PART OF THE MEDULLARY GROOVE OF AN EARLY EMBRYO OF THE GUINEA PIG. (E. A. S.)

ep, folds of epiblast rising up on either side of the middle line, and thus bounding the medullary groove; mg, middle of medullary groove; hy, hypoblast, which is in contact with the medullary epiblast at the middle of the groove, but is elsewhere separated from it by mesoblast, me, which has burrowed forwards between the two primary layers. A cleft is seen in the mesoblast on either side; this is the commencement of the anterior part of the coelom.

Fig. 32.-SECTIONS SHOWING STAGES IN THE CONVERSION OF THE MEDULLARY GROOVE INTO THE
NEURAL CANAL. FROM THE TAIL END OF AN EMBRYO OF THE CAT. (E. A. S.)

ep, me, hy, epiblast, mesoblast, and hypoblast; m.g., medullary groove; n.c. (in IV.), neural
canal; ch, notochord; ca, cœlom; am, tail fold of the amnion.