478 ARTHUR ROBINSON AND RICHAR.D ASSHETON. 



(55), in the mole by Lieberkuhn (35) and Heape (17), in the 

 sheep by Bonnet (5 and 6), and in the bat by Van Beneden 

 (3). In the anterior wall of the opening the chorda and neural 

 epiblast are fused, and laterally and posteriorly the mesoblast 

 hangs in connection with the margins of the streak. 



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 sur- 

 rounding areas. It becomes perforated by an evanescent neu- 

 renteric canal and by a permanent anal orifice. 



In the Cyclostoraata 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 sepa- 

 rates from the superficial epiblast and the hypoblast (Kupflfer, 

 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 mam- 

 mals. It is never perforated by a ueurenteric canal, but 

 the absence of this passage has no important bearing, as it has 

 probably been suppressed. 



