20 Artificial Parthenogenesis and Fertilization 



of the egg into two spheres or cells. The various consecutive 

 stages of this process are depicted in Figs. 10 to 13. First we 

 see that the egg becomes somewhat elongated in the direction 



of the axis of the spindle (Figs. 

 10, 11). This is probably due to 

 the fact that the protoplasm flows 

 to the poles of the spindle and 

 away from its equator. Then 

 there begins a cleavage of the pro- 

 toplasm in the equatorial plane 

 (Fig. 12), until the egg consists 

 of two cells, each of which pos- 

 sesses a nucleus (Fig. 13). This 

 process is called the segmenta- 

 tion of the egg. 

 We will now examine somewhat more closely this process 

 of cell division or cleavage. We know two types of cell division; 

 one corresponds to the type of separation described here. The 



Fig. 9. — Nuclear division (spindle 

 formation) in the egg of S. pur- 

 puratus. 



Pig. 10 Fig. H 



Figs. XO and 11. — Beginning of segmentation In the egg. 



second occurs in plants and consists in the formation of a solid 

 membrane at the equator of the cell, without the two cells 

 actually separating. At a certain stage, both types of divisions 

 are identical, for in both certain materials are carried to the 



