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



The anal membrane is formed in front of the posterior end of the 

 streak in the rabbit — Strahl (55); in the mole — Heape (17); in the 

 guinea-pig — Keibel (29); and in the rat and mouse — .Robinson (43). 

 In the sheep, Bonnet (7) states that the anal membrane is situated at 

 the posterior end of the streak ; and Rabl (41) figures it in the same 

 position in the rabbit. 



The differences described are slight and probably unimportant, and 

 the main facts stand out clearly. As in the Sauropsida, the primitive 

 streak is a line of fusion amidst the germinal layers. It is continuous 

 anteriorly with the neural epiblast and the chorda, laterally and 

 posteriorly with the mesoblast of the surrounding areas. It becomes 

 perforated by an evanescent neurenteric canal and by a permanent 

 anal orifice. 



In the Cyclostomata the posterior part of the blastopore remains 

 open as the anus. In the ventral lip of this orifice the epiblast alone 

 is at first differentiated, the hypoblast and the mesoblast remaining 

 fused until a comparatively late period — Goette (15) and Kupffer(34). 

 The blastopore is closed from before backwards according to Goette's 

 (15) statement, and consequently there is formed in front of the anus 

 a mass of cells called the " Teloblast " by Kupffer (34). This mass of 

 cells remains for a time fused with both epiblast and hypoblast 

 (Goette, pi. iv, fig. 41), but afterwards it separates from the superficial 

 epiblast and the hypoblast (Kupffer, pi. xxviii, fig. 28), but remains 

 continuous in front with the chorda and neural tube. Evidently the 

 area from the front of the anus to the posterior end of the chorda and 

 neural tube in Petromyzon corresponds closely to the anterior portion 

 of the primitive streak of the Sauropsida and mammals. It is never 

 perforated by a neurenteric canal, but the absence of this passage has 

 no important bearing, as it has probably been suppressed. 



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 — " Kupffer '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. Even- 

 tually 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 



