[184] Entwicklungsgeschichte der Unke.
[185] Mélanges Biologiques de l'Académie Pétersbourg, Tome VII.
[186] Archiv. f. mikros. Anat. Vol. VII. p. 114. In the passage on this point Kowalevsky states that in Elasmobranchii the neural and alimentary canals communicate. This I believe to be the first notice published of this peculiar arrangement.
[187] Vide Note on p. [281], also p. [295], and Pl. 9, figs. 1 and 2, and Comparison, &c., Qy. Jl. of Micros. Sci. July, 1875, p. 219. [This Edition, No. VI. p. 125.] These passages give an account of the change of position of the Elasmobranch embryo, and the Note on p. [281] contains a speculation about the nature of the primitive streak with its contained primitive groove. I have suggested that the primitive streak is probably to be regarded as a rudiment at the position where the edges of the blastoderm coalesced to give to the embryos of Birds and Mammals the central position which they occupy.
If my hypothesis should turn out to be correct, various, now unintelligible, features about the primitive streak would be explained: such as its position behind the embryo, the fusion of the epiblast and mesoblast in it, the groove it contains, &c.
The possibility of the primitive streak representing the blastopore, as it in fact does according to my hypothesis, ought also to throw light on E. Van Beneden's recent researches on the development of the Mammalian ovum.
In order clearly to understand the view here expressed, the reader ought to refer to the passages above quoted.
[188] Loc. cit.
[189] Zeitschrift f. Anat. u. Entwicklungsgeschichte, Vol. I. p. 366.
[190] Sitz. der Gesell. zu Marburg, Jan. 1876.