476 ARTHUR ROBINSON AND RICHARD ASSHETON. 



In both the frog and the bird the lateral margins and poste- 

 rior extremity of the streak are continuous with the mesoblast, 

 which lies free between epiblast and hypoblast outside the 

 area of the streak. 



In the frog the anterior wall of the neurenteric canal is 

 bounded by an area of fusion, in which the middle of the 

 neural plate and the posterior end of the chorda are united. 

 After the obliteration of the neurenteric canal the posterior 

 end of the chorda and the centre of the neural plate are con- 

 tinuous with the anterior end of the primitive streak. 



In the bird the anterior end of the primitive streak is at 

 first continuous with the centre of the neural plate and the 

 " Kopflfortsatz/^ and after the formation of the neurenteric 

 canal the chorda and the neural plate are fused in the anterior 

 wall of the latter orifice, becoming again continuous with the 

 anterior end of the primitive streak after the disappearance of 

 the neurenteric canal. 



In the frog the anus is formed in the posterior part of the 

 primitive streak. It is a reopening of a portion of the closed 

 blastoporic orifice. It is not the remains of the blastopore, as 

 in Amblystoma punctatum (39) and Bombinator (15). 

 The anus of the bird is morphologically equivalent to the anus 

 of the frog ; and it is also formed, in all probability, by a re- 

 opening of a previously closed orifice (10). 



In reptiles, according to Kupffer (33), there is no primitive 

 streak, but in the posterior part of the embryonic area the 

 epiblast is invaginated, and the mesoblast arises, in part at 

 least, from the margins of the invagination. The cavity of 

 invagination is the archenteron, and the superficial opening 

 the Urmund, which would thus entirely correspond to the anus 

 of Rusconi in Amphibians. 



But Balfour (1, p. 168), Strahl (54), Hoflfmann (24), Weldon 

 (56), Mitsuruki and Ishikawa (37), and Wenckebach (57) 

 describe a primitive streak. Balfour states that in the primi- 

 tive streak of lizards the epiblast and hypoblast are fused, 

 though the greater part of the streak consists of proliferated 

 epiblast. Wenckebach, however, looks upon the hypoblast as 



