FERTILIZATION. I/I 



of view. 49 As a result of this interaction we find a sexual 

 apparatus of very complex anatomy. But in spite of the 

 great interest of these phenomena, we cannot discuss them 

 now, as they are only of subordinate importance in study- 

 ing the essential nature of the process of fertilization. On 

 the other hand, the nature of this process itself — the mean- 

 ing of sexual generation, must be closely studied. 



In every process of fertilization, as has already been 

 said, two different kinds of cell, male and female, are con- 

 cerned. In animals generally the female cell is called the 

 egg, or egg-cell (ovulum), and the male is called the sperm- 

 cell, or seed-cell (zoospermium, spermatozoon). The female 

 egg-cell, the form and structure of which we have already 

 considered, is in all animals originally of the same simple 

 structure. At first it is simply a globular, naked cell, 

 consisting of protoplasm and cell-nucleus (Fig. 10, p. 134). 

 When this cell lies free, and is capable of motion, it 

 performs a number of slow, amoeboid movements, as we 

 have seen in the case of the egg of the Sponges (Fig. 14, 

 p. 144). But commonly at a later period it is enclosed in 

 peculiar envelopes and coatings of a very heterogeneous and 

 frequently very complex structure. On the whole, the egg- 

 cell is one of the largest of cells. In nearly all animals it 

 is larger than any of the other cells. 



On the other hand, the other cell which plays a part in 



impregnation, the male sperm-cell, is one of the smallest 



cells of the animal body. As a rule, fertilization results 



from a mucous fluid, secreted by the male, coming into 



contact with the egg-cell, either within or without the body 



of the female. This fluid is called the sperm, or male seed. 



The sperm, like the saliva and the blood, is not a simple 

 14 



