60 MANUAL OF ZOOLOGY . SECT, n 



Before the changes begin which lead to the formation of 

 the multicellular Metazoan, another cell, the male cell or 

 sperm, has to unite with the ovum or female cell. Before 

 this takes place, the ovum throws off portions of its 

 substance (Fig. 26, pot.) in the form of two little rounded 

 cells the polar bodies. This preliminary process is 

 known as the maturation of the ovum. The male cell or 

 sperm is a relatively small cell, usually motile, which pene- 



FIG. 25. Ovum of a Sea-Urchin, showing the radially striated cell-membrane, the 

 protoplasm, containing yolk-granules, the large nucleus (germinal vesicle), with 

 its network of chromatin and a large nucleolus (germinal spot). (From Bal- 

 four's Embryology, after Hertwig.) 



trates into the ovum, and coalesces with it the coalescence 

 being what is termed fertilisation or impregnation and 

 the immediate result being that, instead of separate ovum 

 and sperm, we have a compound body, the oosperm^ formed 

 by their union, but not differing at first in any marked 

 degree from the simple ovum, and containing a single 

 nucleus representing the nucleus both of the sperm and 

 of the ovum. 



On impregnation follows the process of segmentation of 

 the oosperm. The nucleus first divides into two ; then the 



