176 REDUCTION OF THE CHROMOSOMES 



period, sometimes even in the cleavage-stages. As development pro- 

 ceeds, each primordial cell gives rise, by division of the usual mitotic 

 type, to a number of descendants known as oogonia (Fig. 87), which 

 are the immediate predecessors of the ovarian egg. At a certain 

 period these cease to divide. Each of them then grows to form an 

 ovarian egg, its nucleus enlarging to form the germinal vesicle, its 

 cytoplasm becoming more or less laden with food-matters (yolk or 

 deutoplasm), while egg-membranes may be formed around it. The 

 ovum may now be termed the o'ocyte (Boveri) or ovarian egg. 



In this condition the egg-cell remains until near the time of fertili- 

 zation, when the process of maturation proper i.e. the formation of 

 the polar bodies takes place. In some cases, e.g. in the sea-urchin, 

 the polar bodies are formed before fertilization while the egg is still 

 in the ovary. More commonly, as in annelids, gasteropods, nema- 

 todes, they are not formed until after the spermatozoon has made its 

 entrance ; while in a few cases one polar body may be formed before 

 fertilization and one afterwards, as in the lamprey-eel, the frog, and 

 Amphioxus. In all these cases, the essential phenomena are the 

 same. Two minute cells are formed, one after the other, near the 

 upper or animal pole of the ovum (Figs. 71, 86); and in many cases 

 the first of these divides into two as the second is formed (Fig. 64). 



A group of four cells thus arises, namely, the mature egg, which 

 gives rise to the embryo, and three small cells or polar bodies which 

 take no part in the further development, are discarded, and soon die 

 without further change. The egg-nucleus is now ready for union 

 with the sperm-nucleus. 



A study of the nucleus during these changes brings out the follow- 

 ing facts. During the multiplication of the oogonia the number of 

 chromosomes is, in some cases at any rate, the same as that occurring 

 in the division of the somatic cells, 1 and the same number enters into 

 the formation of the chromatic reticulum of the germinal vesicle. 

 During the formation of the polar bodies this number becomes 

 reduced to one-half, the nucleus of each polar body and the egg- 

 nucleus receiving the reduced number. In some manner, therefore, 

 the formation of the polar bodies is connected with the process by 

 which the reduction is effected. The precise nature of this process 

 is, however, a matter which has been certainly determined in only a 

 few cases. 



We need not here consider the history of opinion on this subject 

 further than to point out that the early observers, such as Purkinje, 

 von Baer, Bischoff, had no real understanding of the process and 

 believed the germinal vesicle to disappear at the time of fertilization. 



1 See, however, p. 194. 



