THE DEVELOPMENT OF THE FROG 255 



the previous figure. The great increase of the size of the 

 enteron has resulted in a corresponding diminution of 

 '. the blastoccele, which is now displaced to the left-hand 

 side of the picture, but can be traced between the outer 

 and inner limbs of the fold right round to the dorsal lip 

 of the blastopore. 



It must be borne in mind that while the blastopore is 

 advancing it is also elongating and becoming crescentic in 

 shape, and that the horns of the crescent bend inwards and 

 eventually meet together. The same process of infolding 

 that is observed at the dorsal lip is also in progress, but at 

 a slower rate, along the horns of the crescent, and eventually, 

 when the horns meet and unite, the infolding extends all 

 round a ring and we get the condition represented in fig. 

 59, G (corresponding to stage G in fig. 58), where the ventral 

 lip of the blastopore is also seen to be infolded and has 

 advanced from the equator towards the lower pole of the 

 egg to meet the dorsal lip, the two being separated by the 

 yolk-plug i.e. by the circular space where the white cells 

 still project on the surface. It can be seen that the enteron 

 has increased in size and that the blastoccele is reduced to 

 very narrow dimensions. 



The result of this complex process of overgrowth and en- 

 folding is the obliteration of the blastoccele and the formation 

 of a new cavity, the enteron, whose roof and side walls are 

 formed by two layers, each several cells deep, and whose floor 

 is formed by a compact mass of yolk-cells. The external layer 

 of cells is the epiblast, differing only from that of the embry- 

 onic Amphioxus in being more than one cell thick. The 

 internal layer .forming the roof' and side walls of the enteron 

 may be called the lower layer or primitive hypoblast, and is to 

 be compared with the interior layer of the gastrula of Amphioxus. 

 The mass of yolk-cells has no exact counterpart in Amphioxus, 

 but a little consideration will make it clear that the stage which 

 we are now considering in the frog is really a gastrula stage, 

 and that the changes that have led up to it are really a process 

 of invagination, modified by the presence of a mass of inert 

 yolk-cells, which have prevented a complete tucking in of one 

 hemisphere within the other. Further differences may be 

 noted between the two forms. In Amphioxus the process of 

 invagination is relatively rapid, in the frog it is slow, being 



