112 



TEE POPULAR SCIENCE MONTHLY 



structure and function have arisen — a subject to which we shall return 

 later. But the developmental processes which lead to the formation of 

 fully developed ova and spermatozoa come to a full stop before fertili- 

 zation and they do not usually begin again until a spermatozoon has 

 entered an ovum, or until the latter has been stimulated by some other 

 outside means. In some animals and plants eggs may develop regularly 

 without fertilization, the stimulus to development being supplied by 



l st Mat.Sp> 



<SN'-' 



3n\c 



-2^ Mat 



Sp 



Si~2& PB 



Fig. 23. Diagrams of the Maturation and Fertilization of the Egg of a 

 Molldsk (Crepidula). A, B. First maturation division (1st Mat. Sp.) G. Second 

 maturation division (2d Mat. Sp.) and first polar body (1st PB) resulting from first 

 division. gN, sperm nucleus, £0, sperm centrosome. D. Approach of sperm nucleus 

 (<$N) and sperm (£S) to egg nucleus (<£N) and sphere (<j>S) ; the second polar body 

 (ZdPB) has been formed and the forest has divided (is* PB). E. Meeting of egg and 

 sperm nuclei and origin of cleavage centrosomes. F. First cleavage of egg showing 

 direction of currents in the cell. 



