74 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS 



mammals, it may enter at any point. Entrance is effected 

 partly by the boring motion of the spermatozoon, but 

 partly, in some cases at least, by an active engulfing action 

 on the part of the egg ; the whole process is usually com- 

 plete within one minute. In the most studied case, the 

 sea-urchin egg, the first visible change in the egg follows 

 almost immediately : the delicate vitelline membrane 

 becomes detached from the surface of the egg to form the 











Fig. 37- — Diagram showing relative size of an egg-cell and 



a sperm -cell (S.). 



A'., Nucleus ; C, colloidal cytoplasm ; CHR., portions of 

 chromosomes ; G.S., the nucleolus, which used to be 

 called the germinal spot. 



fertilisation membrane ; the space between this and the 

 egg itself is filled with a clear fluid. There is a striking 

 increase in the permeability of the surface of the ovum 

 during these changes, so that dissolved substances in the 

 cytoplasm may escape altogether and be lost, and the egg 

 is most sensitive to abnormalities in the external medium. 

 Moreover, there is a great increase in the metabolism of 

 the ovum after fertilisation : increased consumption of 

 oxygen, increased evolution of COg, and increased heat 

 production. 



