110 



DEVELOPMENT OF THE FROG 



Two more vertical clefts next appear simultaneously at 

 the upper pole, midway between the two primary clefts, and 

 extending downwards divide first the smaller and then the 

 larger cells, giving sixteen cells in all, eight smaller upper 

 ones, and eight larger lower ones. Two more horizontal clefts 

 then appear, which again double the number of segments, 

 giving thirty-two in all. 



From this stage segmentation proceeds in a less regular 

 manner, the upper and smaller cells dividing more rapidly than 

 the lower and larger ones. By means of radial and concentric 



FIG. 2i. Segmentation of the frog's egg. (From Haddon, after 

 Ecker.) 



The numbers above the figures indicate the number of segments at 

 the several stages. The dotted lines mark the positions of the clefts 

 that will next appear. 



clefts, the number of cells is rapidly increased, division of the 

 cells being in all cases, as from the first, preceded by division 

 of their nuclei. 



At the stage when only eight cells are present, i.e., on the 

 completion of the third cleft, a small cavity appears in the 

 centre of the egg, round which the cells are grouped : during 

 the later phases of segmentation this segmentation cavity 

 or blastocoele, as it is called, increases considerably in size : 

 it is from the first situated nearer the upper than the lower 

 pole of the egg, and is filled with fluid. 



At the close of segmentation the egg has the structure 

 shown in section in Fig. 12. 



