NOTES ON ELASMOBRANCH DEVELOPMENT. 565 
before the anterior [what I have called posterior] and more 
ventral part, which is represented by the uncovered portion of 
the yolk.” 
I have dwelt at some length upon this point because Balfour’s 
description of the Elasmobranch blastopore has always bothered 
me, in that it does not show the connection between the yolk 
part of the blastopore—the linear streak—with the dorsal 
part ; and also because I wish to present a slight modification 
of the comparison which Balfour made between the primitive 
streak of the Amniota andthe linear streak on the Hlasmo- 
branch yolk. Balfour does not say that the two structures are 
homologous ; he expressly guards himself from this. He says 
(‘Comparative Embryology,’ Ist ed., vol. ii, ch. 111, p. 51), “A 
linear streak [my woodcut, ef] formed by the coalesced edges of 
the blastoderm is left connecting the embryo with the edge of 
the blastoderm. This streak is probably analogous to (though 
not genetically related with) the primitive streak in the 
Amuiota” (the italics are mine). But he undoubtedly does 
compare the primitive streak with this linear part of the yolk- 
blastopore of Elasmobranchs ; and he says (‘Comparative Hm- 
bryology,’ vol. ii, Ist ed., ch. xi, p. 240), “That it (primitive 
streak) is in later stages not continued to the edge of the 
blastoderm, as in Elasmobranchii, is due to its being a rudi- 
mentary organ.” 
The modification which I would propose to suggest in the 
comparison is as follows. The primitive streak of the Am- 
niota is, as is well known, partly involved in the tail fold, and 
tucked under on to the ventral surface of the embryo. It 
thus becomes divided into a dorsal part, at the front end of 
which is the neurenteric canal or its rudiment, and a ventral 
part. The dorsal part is in birds for some time placed in a 
dilated posterior part of the still open medullary groove called 
the sinus rhomboidalis. This part I would compare to 
the dorsal part of the blastopore shown in the same position 
and relations in my figs. 3 and 4. The ventral part, on the 
other hand, I would compare to the part of the blastopore 
which in Elasmobranchs runs along the ventral side of the tail 
