FORMATION AND FATE OF THE PRIMITIVE STREAK. 107 



stated. The fused area corresponds in the Teleostei even more closely 

 than in the Cyclostomata with the anterior portion 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 condition of 

 the posterior end of the embryro in the Elasmobranchii (p. 88), 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 subse- 

 quently 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 complete 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. 222) 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 temporaria. 



The primitive streak of the Amphibia appears during the period of 

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

 layers in the lips of the blastopore and by concrescence of the 

 lips of the blastopore, either from before backwards, as in Triton 

 (Goette, 15), or by fusion of the lateral lips of the blastopore 

 between the neurenteric canal and the anus, arriving at its completion 

 on the obliteration of the neurenteric canal, as in Bombinator (15) 

 and Amblystoma punctaUim (39) or it may be formed, as we have 

 already shown in Rana temporaria, by fusion of the lips of the 

 blastopore from behind forwards. 



In Teleosteans and Cyclostomes it is formed by fusion of the 

 blastoporic lips from before backwards, also whilst the archenteron is 

 being formed and extended, and in Elasmobranchs by fusion of the 

 lateral lip of the blastopore behind the neurenteric canal. 



In the Sauropsida and Mammalia we have no definite proof of a 



