FATE OF BLASTOPORE, ETC., IX CHELONIA. (^g 



doe.s not say that the two structares* are liomolog-ous ; he expressly 

 o-uards himself from this. He says (' Comparative Embrvoloo-v,' 

 1st ed., vol. ii, cli. iii, p. 51). ' A linear .streak [my woodcut,** e f] 

 formed bv the coalesced edo-es of the blastodenu is left ccninectinc- 

 the embryo with the edge of the blastoderm. This ^^treak is probably 

 analogous to (tltoiKjli not (jcncticaUij related iritli) the primitive streak in 

 the Amniota' (the italics are mine). Ihit lie undoubtedly does com- 

 pare the primitive streak with tins linear part of the yolk-blastopore of 

 Elasmoljranchs ; and he says (Comparative Embrvologv, vol. ii, 1st 

 ed., ch. xi, p. 240), ' That it (primitive streak) is in later stages not 

 continued t(3 the edge of the blastoderm, as in Ehismobranchii, is due 

 to its being a rudimentary organ.' The modification which I would 

 prop(3se to suggest in the C(3mparison is as follows. The primitive 

 streak of the Amniota is, as is well known, partly involved in the tail 

 fold, and tucked under on to the ventral surface of the embryo. f It 

 thus bec(3mes divided into a dorsal part, at the front end (jf which is 

 tlie neurenteric canal or its rudiment, and a ventral part. The dorsal 

 part is in birds foi- some time ])]aced in a dilated posterior part of the 

 still open medullary groove called the sinus rhonilniidalis. This part I 

 would compare h) the dorsal part of the l)lasto[M)re shown in the 

 same position and relations in my figs, o and 4. The ventral 

 part, on the otlier liand, I would compare to the [)art of the 

 blastopore which in Elasmobranchs runs along the ventral side 

 of the tail to the yolk-stalk (my Woodcut c (l);X while the linear 

 part of the yolk-blastopore in Elasmcnd^ranchii (mv Woodcut, 6 /") is 

 unrepresented or rudiinentanj in. Aces and Autniota (jenerallij — is, in fact, 



* I. e., the primitive streak of the Amniota and the linear streak on the Elasmohranch yolk. 

 ** Copied in Woodcut XI [ of the present article. 



t In view ..f the facts l^rought out in the present article in rega.rd to the tail formation, I 

 should object to the words " tucked under" which Sedgwick uses. 

 X Copied in Woodcut XII of the present article. 



