240 ADAM SEDGWICK. 



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 and the linear streak on the Elasmo- 

 branch yolk. Balfour does not say that the two structures are 

 homologous ; he expressly guards himself from this. He says 

 C Comparative Embryology/ 1st ed., vol. ii, ch. iii, 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 

 Amniota'' (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 Em- 

 bryology/ vol. ii, 1st 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 



