FORMATION AND FATE OF THE PRIMITIVE STEEA.K. 479 



In Teleostean fishes an area of fusion is found round the lips 

 of the blastopore (Henneguy, 18), but the fusion in front of 

 the blastopore is much more extensive than that on the sides 

 and posteriorly, and in its anterior part a vesicle appears — 

 " Kupflfer's vesicle." In front of Kupffer's vesicle the chorda 

 and neural plate are fused, and the margins of the fused area 

 are continuous with the mesoblast. Eventually the posterior 

 portion of the blastopore is closed by the fusion of its lips 

 (18, p. 502). The position of the anus is not definitely 

 stated. The fused area corresponds in the Teleostei even 

 more closely than in the Cyclostomata with the anterior por- 

 tion of the primitive streak of the Sauropsida and mammals, for 

 it is partially perforated anteriorly by Kupffer's vesicle, which 

 is evidently situated in the position of the neurenteric canal of 

 the higher Vertebrata. 



We have before referred to Balfour's description of the con- 

 dition of the posterior end of the embryo in the Elasmo- 

 branchii (p. 11), and to this we have only to add that at a later 

 period the ventral part of the blastopore also becomes closed 

 by fusion, and that at a subsequently later period the anus is 

 formed as a secondary opening in the line of fusion (Schwarz, 

 47). 



The observations upon the Ganoids are not sufficiently com- 

 plete to afford any definite basis for comparison, but it appears 

 (see Balfour, vol. ii, pp. 84 — 86, and the extract in Hoffmann 

 and Schwalbe's ' Jahresbericht ' for 1878, p. 223) that after the 

 segmentation and during the formation of the archenteron the 

 blastopore becomes distinct, first at its dorsal lip and then in 

 its whole circumference ; it ultimately closes, and Salensky (44) 

 states that the anus is produced afterwards in the situation 

 which was first occupied by the blastopore. There is, however, 

 no evidence as to whether the blastopore in Ganoids closes 

 from before backwards, as in Teleostei and Elasmobranchii and 

 Cyclostomes, or from behind forward as in the Rana tera- 

 poraria. 



The primitive streak of the Amphibia appears during the 

 period of extension of the archenteron. It is formed by a fusion 



