ANIMAL BIOLOGY. 



[Part I. 



which star-like striations extend into the protoplasm of the cell 

 (Fig. 31, ii.). lii the middle of the spindle the fibres of the 



reticulum aggregate so as 

 to form the equatorial plate. 

 This splits in the equatorial 

 line into two plates which 

 travel away from each other 

 towards the poles, the proto- 

 plasm between them having 

 a striated appearance (Fig. 

 31, iii.). Meanwhile the 

 protoplasm of the general 



FIG. 31. KARYOKINESIS. 



substance of 

 e.g. so far 



the cell becomes 

 as nearly to divide 



constricted, in Fig. 31, iv., 

 the cell into two, and even- 



tually to such an extent as to cause its complete division into 

 two cells, each of which now contains a nuclear spindle, 

 which may either relapse into a reticulum or straightway re- 

 divide. Watched under the highest power of the microscope 

 the fibres of the equatorial plate are seen to group themselves 

 into the form of a rosette, which then, with sundry other 

 changes of form, separates into two rosettes, each of which forms 

 a new equatorial plate. Such are the internal changes in 

 the segmenting cells. We must now turn to the consideration 

 of the segmentation of the ovum as a whole, and the formation 

 of the so-called germinal layers to which this process gives rise, 

 remembering that each case of cell-division is accompanied by 

 karyokinesis. But before describing what takes place in frog, 

 fowl, or rabbit it will be well to pave the way by taking an 

 ideal case. 



We will suppose that the ovum contains no food-yolk. It 

 first divides into two equal halves, and then each of these again 

 into two. We may call the points of intersection of these two 

 planes the poles, and the planes vertical planes. We thus have 

 four cells produced by two vertical planes, the segmentation 

 being so far similar to that shown in Fig. 33. The next piano is 

 equatorial midway between the poles. By this plane the four 

 cells are subdivided into eight. Then follow two more vertical 



