THE OVUM 



95 



^'g- 43- — Epgs of the annelid Nereis, before aiui after fertilization, X 400 (for intermediate 

 stages see Fig. 71). 



A. Before fertilization. The large germinal vesicle occupies a nearly central position. It con- 

 tains a nrivvork of chromatin in which are seen five small darker bodies; tliese are the quadruple 

 chromosome-groups, or tetrads, in process of formation (not all of them are shown) ; these alone 

 persist in later stages, the principal mass of the network being lost ; g.s. double germinal spot, 

 consisting of a chromatic and an achromatic sphere. This egg is heavily laden with yolk, in the 

 form of clear deutoplasm-spheres {d) and fat-drops (/), uniformly distributed through the cyto- 

 plasm. The peripheral layer of cytoplasm (peri-viielline layer) is free from deutoplasm. Outside 

 this the membrane. B. The &^g some time after fertilization and about lo divide. The deuto- 

 piasm is now concentrated in the lower hemisphere, and the peri-vitelline layer has disappeared. 

 Above are the two polar bodies (p.b.). Below them lies the mitotic figure, the chromosomes 

 dividing. 



