252 BIOLOGY 



cies. Essentially the same series of events occurs in all animals 

 where a fertilization takes place, although the order of events 

 is not always the same. In a previous chapter we have seen 

 that in all animals, when the chromatin of the nucleus breaks 

 into chromosomes before division, the number of chromosomes 

 is always the same in all cells of the species. In order to 

 illustrate the process of the origin and union of the sex cells, 

 we will describe the process in an animal that has four 

 chromosomes, meaning by this that all of the cells of the ani- 

 mal (except the germinal cells to be described) contain four 

 chromosomes at the time when cell division takes place. 



Origin of the Egg (Oogenesis). The egg is simply one of 

 the ordinary cells of the ovary. During the early life of the 

 animal, the cells in the ovary increase by the ordinary process 

 of cell division, with nothing especial to distinguish it from 

 the cell division of the other cells. In all cases, the cells are 

 about the ordinary size and all contain the normal number of 

 four chromosomes. This process continues indefinitely during 

 the early life of the animal, until it is ready to produce eggs. 

 When this time comes, some of the cells of the ovary begin 

 to increase greatly in size, and become in a short time very 

 much larger than the ordinary cells, not only than the cells 

 of the body generally, but much larger than all of the other cells 

 in the ovary. This increase in size is due largely to deposition 

 in the egg of food material which is to serve as nourishment 

 for the young that is subsequently to develop from the egg. 

 At the time the egg increases in size, a peculiar change takes 

 place in the chromosomes within the nucleus. By a series 

 of divisions, this chromatin divides into a number of chromo- 

 somes which is always double that found in the ordinary cells 

 of the animal. In our illustration, instead of four of these chro- 

 mosomes, there are eight. These chromosomes always assume 

 at this stage the arrangement in groups of fours, such as is 

 shown in Figure 121 A. There is thus produced a large pri- 

 mary egg (Gr. don = egg-fci/tos), called an oo'cyte, containing 



