No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 369 



The lateral extension of the groove soon reaches its limit, 

 the ends turn toward the vegetative pole, and in 1 5 hrs. the 

 crescentic blastopore has extended to the form shown in PI. 

 XX, Fig. 3. This is soon followed by the union of the ends 

 completing the blastopore. At this time there in an infolding 

 around the entire rim, the rate being most rapid at the dor- 

 sal margin, and gradually decreasing toward the ventral. 



Fig. 7 represents a condition often observed where the 

 blastodermic margin is defined at a much earlier period than in 

 Fig. 3. A like condition was often found in Rana (Fig. 8) and 

 in Petromyzon (Fig. 10). 



The closure of the blastopore progresses gradually with the 

 infolding. In 18 hrs. the condition shown in Fig. 4 is reached, 

 when the external indications of gastrulation are beginning to 

 fade. The blastopore has narrowed as a result of the unequal 

 infolding. I am also inclined to believe that it is in part the 

 result of mechanical pressure caused by the thickenings of the 

 epiblast {n.f) on either side of the dark hemisphere. 



In a few hours the oval opening shown in Fig. 4 has been 

 reduced to the slit-like aperture indicated in Fig. 5. This nar- 

 row slit is soon entirely closed at its center (Fig. 6), leaving an 

 anterior opening lying just within the neural folds {n.f.), form- 

 ing the neuropore («/.), and a posterior just without which 

 persists and forms the anus {a) as stated by Morgan ('SO). This 

 in Amblystoma is the usual method of closure, but it is by no 

 means the only method. I have often observed a pear-shaped 

 opening with the smaller end anterior, instead of the more 

 usual dumb-bell outline. I have also followed the closure of 

 the blastopore in Rana palustris, and am able to confirm the 

 observations of Ziegler ('92), that the posterior portion of the 

 blastopore persists as the permanent anus. 



Hertwig ('92) states that in the Newt the " Urmundrander 

 sich in einer von vorn nach hinten langsam fortschreitenden 

 Richtung in der Medianebene zusammengelegt haben und 

 verschmolzen sind." 



Robinson and Assheton ('91) find in Rana that " the anus of 

 Rusconi gradually diminishes in size by the concrescence of 

 the ventral parts of the lateral lips." 



