[12] Kowalevsky (“Beiträge zur Entwicklungsgeschichte der Holothurien,” Mémoirs de l'Ac. Imp. de St Petersbourg, vii ser., Vol. XI. 1867) describes the division of nuclei during segmentation in the Holothurians, and other observers have described it elsewhere.
[13] Götte, at the end of a paper on “The Development of the Layers in the Chick” (Archiv. für Micr. Anat., Vol. X. 1873, p. 196), mentions that the so-called cells in Osseous fishes which Oellacher states to have migrated into the yolk, and which are clearly the same as those mentioned by Owsjannikow, are really not cells, but large nuclei. If this statement is correct the phenomena in Osseous fishes are precisely the same as those I have described in the Dog-fish.
[14] This has been already made out by Kowalevsky, “Würmern u. Arthropoden,” loc. cit.
[15] This groove is the only structure which it seems possible to compare with the so-called “primitive groove” of Birds. It is, however, doubtful whether they are really homologous.
[16] For the specimens of this age I am indebted to Professor Huxley.
[17] If Romiti's observations (Archives für Mikr. Anatom. Vol. IX. p. 200) are correct, then the ordinary view of the Wolffian duct arising in Birds as a solid rod at the outer corner of the protovertebræ will have to be abandoned.
[18] While correcting the proofs of this paper I have come across a memoir of W. Müller (“Ueber die Persistenz der Urniere bei Myxine Glutinosa,” Jenaische Zeitschrift, Vol. VII. 1873), in which he mentions that in Myxine the upper end of the Wolffian duct communicates by numerous openings with the pleuro-peritoneal cavity; this gives to the suggestion in the text a foundation of fact.
VI. A Comparison of the Early Stages in the Development of Vertebrates[19].
With Plate 5.