CHAPTER II 

 THE BEGINNING OF DEVELOPMENT 



Egg and Sperm 



The history of a developing animal really begins 

 when the egg-cell becomes fertilized by uniting with 

 a sperm-cell, but before this can happen there must 

 occur a very important series of processes by which 



NUCLEUS 



... HEAD 



O" \ Y- CENTREPIECE 



- TAIL 



a 



Fig. I. — Diagram of an Egg-cell {a) and a Sperm-cell {b). The 



sperm is more highly magnified; in Man the c^g% is o-2 mm. in 



diameter and the sperm about 0-05 mm, or i /500th inch long, 



including the tail 



these cells are elaborated. Egg- and sperm-cells are 

 known collectively as germ-cells^ and like other cells 

 they consist of a mass of living matter or protoplasm, 

 containing a nucleus. But in the details of their 

 structure (Fig. i) they are very speciahzed and 

 unlike normal cells, as might be expected from the 

 extraordinary things they have to do. The egg-cell 

 is usually rather large as cells go, since it has to 



