ORGAN-FORMING MATERIALS IN FROG'S EMBRYO. 1 29 



drawn to larger scale than the preceding. Since the cells are 

 dividing during this time without the number of layers in the roof 

 of the blastoccel becoming correspondingly greater in fact they 

 are fewer in the gastrulation stages this must mean that as the 

 cells increase in number they push out towards the sides, and in- 

 crease the material of the embryo-ring, which is, pari passu, 

 pushing below the equator of the egg. 



Rana sylvatica. Owing to the looseness of the cells of the 

 roof of the segmentation cavity in this species - - whether repre- 

 senting a real condition or due to the effects of the preserving 

 reagents I do not know the preparations show less well the 

 thinning out of the roof of the blastocoel. The first stage figured, 

 Fig. 1 6, is a section through a 32-cell stage. The inwandering of 

 the upper cells has not yet been accomplished, the segmentation 

 cavity is well developed, and its roof is thick. The upper ends 

 of the four lower cells of the eight-cell stage are, at this time, cut 

 off by the fifth divisions, that usually lies obliquely. In the figure 

 these cells are represented by the two large cells at the sides. 

 They contribute, I think, much of the material that forms the 

 " embryo." 



The next figure, Fig. 17, shows a later stage, cut somewhat 

 obliquely. The blastoccel is large, andjts roof thinner, but the sides 

 are as thick as before, or nearly so, as shown by superposing the 

 32-cell stage upon this one. The next two figures, Figs. 18-19, 

 show a broadening of the blastoccel, and thinning of its roof, accom- 

 panied by a downgrowth of the small cells around its sides. The 

 next figure, Fig. 20, shows the downgrowth carried further, and 

 the yolk floor correspondingly lifted up. The last figure, Fig. 21, is 

 through the dorsal lip of the blastopore, at the time when the invagi- 

 nation has just appeared. The roof of the segmentation cavity is 

 thinner and continuous at the anterior end with the ectoderm in 

 front of the dorsal lip. This ectoderm is now separated from the 

 cells inside ; except in the dorsal lip itself, where ectoderm and 

 mesoderm completely coalesce. 1 A tongue of small cells, run- 

 ning upwards from the blastopore beneath the ectoderm, repre- 

 sents the mesoderm. This mass of cells along with the yolk- cells 



1 Possibly ectoderm cells are added along the middle line to the sheet of mesoderm 



and give rise to the notochord. 



