EGGS AND SPERMS -55 



egg proper, while the shell and the white are merely 

 protecting and nourishing structures about it. The yolk 

 is a single very large cell. 



Eggs usually have large nuclei, and a good supply of 

 protoplasm. They are without power of motion. They 

 vary in size from that of the ostrich down until they 

 are too small to be seen with the naked eye. There is 

 little connection between the size of the animal or plant 

 and the size of the egg it produces. The eggs of humming 

 birds are many hundred times as large as those of trees 

 or of human beings. Eggs are plump, well-nourished, 

 and sluggish cells. They need to be stimulated to make 

 them active. 



5. The Development of Eggs. It was said a moment 

 ago that the egg is a cell. So it is, but there is another 

 fact that must be mentioned about it. It is not just like 

 the cells of the body from which it comes. In order to 

 make this clear we must recall a few facts about the 

 structure of cells. In each cell, aside from the more fluid 

 protoplasm, (cytoplasm) there is a definite protoplasmic 

 body called the nucleus. In this nucleus there is a very 

 important substance called chromatin. As the time 

 approaches for a cell to divide, this chromatin tends to 

 collect into a number of rod-like bodies known as 

 chromosomes. The number of these in the body cells of 

 a given species of plants or animals is practically constant. 

 These chromosomes behave in a very interesting way in 

 the development of an egg. 



Let us suppose that the nucleus of the ordinary cells 

 of an animal body that produces an egg has sixteen of 

 these chromosomes. By a process which we will not 

 study in detail here, the nucleus of the egg cell loses just 

 one-half its chromosomes as it prepares for fertilization. 

 In the supposed case the nucleus of the ripe egg cell 

 would contain eight chromosomes instead of the sixteen 

 which is characteristic of the other cells. 



