CHAPTER II 



The initiation of development: 

 The fertilisation of the egg 



In sexual reproduction, which occurs in all multicellular 

 animals, two types of germ cells or gametes are produced by 

 the adult individuals. In some cases both types are produced 

 by the same individual, in other cases they are formed by 

 different animals. The two types are female germ cells, or eggs, 

 and male germ cells, or sperms (spermatozoa). Each egg fuses 

 with a sperm; this is the process of fertilisation. The fertilised 

 egg, or zygote, develops into a new individual. 



As a rule, the egg is a very large cell, consisting of protoplasm 

 with a large nucleus, commonly called a germinal vesicle. In 

 the protoplasm, a great quantity of food, in the form of protein 

 and fat globules, is often accumulated. This is the so-called 

 yolk, which constitutes a store, given to the young individual 

 as it sets out upon its path of life. Its supplies the energy for 

 the first stages of development, during which the embryo can- 

 not yet take up food from its environment. The eggs originate 

 from small cells, the so-called oogonia, in the ovary of the 

 maternal animal. They swell strongly in the course of the 

 accumulation of the yolk substances, and grow into immature 

 eggs, or oocytes. 



In contrast with the eggs, the sperms are very small, usually 

 thousands of times smaller than the egg; they do not contain 

 any yolk. Apart from slight changes in shape, eggs as a rule are 

 immobile ; sperms, on the other hand, have the power of locomo- 

 tion. This enables them to move actively towards the eggs, 

 and to penetrate into them, which results in fertilisation. The 

 shape of the sperms varies rather considerably among the differ- 



