104 ARTHUR ROBINSON, M.D., AND RICHARD ASSHETON, M.A. 



(47), from behind forwards according to Balfour and Deighton (2) and 

 Roller (30) ; and the appearance is due apparently to thickening and 

 fusion of the two primary layers — Duval (10), Balfour and Deighton 

 (2). 



In both the frog and the bird the lateral margins and posterior 

 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 continuous with the anterior end of the primitive 

 streak. 



In the bird the anterior end of the primitive streak is at first con- 

 tinuous with the centre of the neural plate and the " Kopffortsatz," 

 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 Ambly stoma 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 reopening of a previously closed orifice (10). 



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

 but in the posterior part of the embryonic area the epiblast is in- 

 vaginated, 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), Hoffmann (24), Weldon (56), 

 Mitsuruki and Ishikawa (37), and Wenckebach (57) describe a 

 primitive streak. Balfour states that in the primitive 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 a purely passive agent in the formation 



