THE BEGINNING OF DEVELOPMENT 27 



contain food material for the embryo to use before 

 it develops a digestive apparatus. This food material 

 is stored as grains of yolk, which are scattered 

 through the cytoplasm, that is to say, all the proto- 

 plasm outside the nucleus. As it is fairly heavy, the 

 yolk collects at the bottom of the ^gg, which is 

 therefore stratified, with a yolk-laden vegetative pole 

 below and a non-yolky animal pole at the top. The 

 nucleus usually lies near the top, in the clear 

 protoplasm of the animal pole. Non-yolky eggs are 

 often not very much bigger than other cells; the 

 human tgg, for instance, is about o • 2 mm. or a 

 hundredth of an inch in diameter. But when there 

 is much yolk, the egg-cell may be swollen to an 

 enormous size. The "yolks" of birds' eggs are single 

 cells, the biggest known, with only a tiny little patch 

 of cytoplasm nearly hidden in the huge mass of 

 yolk. The sperm-cell is still more highly speciahzed. 

 It consists of three parts : the head which contains 

 the nucleus, the centre-piece, and a long tail which 

 beats to and fro and drives the sperm actively about 

 through the fluid in which it exists. Sperm-cells are 

 very small, containing no yolk and hardly any cyto- 

 plasm, and their light construction enables them to 

 move about comparatively rapidly. A human sperm 

 can travel at the rateof about an inch in three minutes. 

 Eggs, on the contrary, are rarely able to move. 



The most important part in the elaboration of the 

 germ-cells is the preparation of the nucleus, and this 

 process is essentially the same both in the eggs and 

 in the sperm. For most of the time the nucleus of 



