SEGMENTATION OF THE OVUM. 



21 



complement of chromosomes, and to furnish the stimulus required to inaugurate the 

 karyokinetic cycle of segmentation. 



Segmentation of the Ovum. — The union of the male and female pronuclei 

 and the resulting formation of the segmentation nucleus is followed immediately 

 by the division of the ovum into two new elements ; each of these gives rise to two 

 additional cells, which, in turn, produce following generations of segmentation cells, 

 or blastoineres. This process of repeated division of the fertilized ovum and its 



Fig. 17. 

 ABC 



D 



F 



H 



I 



Early stages of segmentation as seen in sections of ova of mouse. X 500. (Sobotta.) A-D show the rearrange- 

 ment of the chromosomes contributed by the male (»/) and female (/) pronuclei as preparatory to the first cleavage 

 of the fertilized ovum; p,p, polar bodies; ep, stage of equatorial plate; a, A, daughter groups of chromosomes. 

 E, F, the daughter cells arising from first cleavage. G', one cell (b) is larger and is preparing to divide. H, later 

 stage of this division. /, stage of three segmentation spheres (a and c, c) resulting from this division. 



descendants constitutes segmentation, — a process common to the development of 

 all animals and plants above the very simplest. 



Study of the details of segmentation in the various classes of animals shows 

 that a close relation exists between the character of the cleavage and that of the 

 ovum with regard to the amount and distribution of the nutritive yolk, or deuto- 

 plasm, present. 



In the human and mammalian e^r^ the nutritive yolk particles are compara- 

 tively meagre and are uniformly distributed throughout the vitellus ; in such eggs 

 there is no aggregation of the food particles, hence such ova are termed Iwinolecithal 

 or with a homogeneous yolk. In the eggs of birds, reptiles, and fishes, on the con- 



