60 MANUAL OF ZOOLOGY sect. 



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, pel) in the form of two little rounded bodies — 

 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 penetrates into 



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

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

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

 four's Embryology, after Hertwig.) 



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 both the nucleus of the sperm and that of the 

 ovum. 



On impregnation follows the process of segmentation of 

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



