ON THE FATE OP THE BLASTOPORE IN THE NEWT. 169 



mation of the medullary folds. In fig. 2 the dorsal half of 

 the epiblast is seea to be thickened. This occurs first in the 

 anterior part of the body, where also the folds are first clearly 

 formed. They consist of a pair of sharply-marked ridges, 

 bounding a very wide, flat area. The medullary plate, which 

 includes the whole of the dorsal surface, is made up of narrow 

 deep columnar cells. The rest of the epiblast, which formerly 

 consisted of a single layer of columnar cells (vide figs. 1, 3), 

 now begins to divide into two layers of flatter cells. These 

 well-known peculiarities of the medullary plate and general 

 epiblast have already been sufficiently figured by previous 

 observers. 



The primitive groove at this period extends from the blasto- 

 pore throughout the whole medullary plate. The consequent 

 division of the latter into two halves is especially conspicuous 

 in front. It occasionally happens that the groove is absent in 

 the middle region of the body. This was the case with the 

 embryo, transverse sections of which are represented in figs. 3, 

 4, 5, and in which the medullary folds existed anteriorly, but 

 diminished gradually and vanished behind. Fig. 3 shows the 

 open blastopore, with the three embryonic layers coalescing at 

 its edges in the ordinary manner. In fig. 4 the rounded pri- 

 mitive groove is seen indenting the primitive streak. In fig. 5 

 the groove is flatter, but the layers are still fused beneath it. 

 The blastopore itself at this stage is narrow and elongated. 



In another specimen of the same stage as that just described 

 I find that the primitive groove extends for a short distance i n 

 front of the medullary folds. Near its anterior end it becomes 

 rather suddenly considerably deeper and also loses its rounded 

 outline, being instead triangular in section and sharply pointed 

 at its apex. It presents in this region, in fact, an appearance 

 strikingly similar to that of the blastopore, although not com- 

 municating with the archenteron. I believe, however, that the 

 epiblast and hypoblast are fused at this point, and it can 

 hardly be doubted that this deep pit, with the fused layers at 

 its apex, represents the front end of the blastopore. It is 

 evidently the same structure as the pit found at the front end 



13 § 



