118 



DEVELOPMENT OF THE FBOG. 



into two completely separate halves along a plane corresponding 

 with the vertical plate mentioned above. 



Each of the two nuclei soon divides again into two, and a 

 second cleft is formed in the same manner as before : it also is 

 vertical, but in a plane at right angles to the first one ; and on 

 its completion the egg consists of four precisely similar segments 

 or cells, each with a nucleus. 



The third cleft is horizontal, but not equatorial, lying nearer 

 the upper than the lower pole : it divides each of the four cells 

 into two, an upper smaller and a lower larger one. 



Two more vertical clefts next appear simultaneously at the 

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

 ing 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. 

 i 2 4 s 



Fig. 22. 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. 



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 

 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, as it 

 is called, increases considerably in size : it is from the first 



