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 18. 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- 
Fic. 9.—Nuclear division (spindle sesses a nucleus (Fig. 13). This 
formation) in the egg of S. pur- process is called the segmenta- 
puratus. 
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. 10 rig. 11 
Fries. 10 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. Ata certain stage, both types of divisions 
are identical, for in both certain materials are carried to the 
