236 REDUCTION OF THE CHROMOSOMES 



divisions occur in all the higher forms (Muscinese, pteridophytes, and 

 phanerogams), and in some, at least, of the lower ones. Here, how- 

 ever, the phenomena are complicated by the fact that the two divi- 

 sions do not as a rule give rise directly to the four sexual germ-cells, 

 but to four asexual spores which undergo additional divisions before 

 the definitive germ-cells are produced. In the flowering plants there 

 are only a few such divisions, which give rise to structures within the 

 pollen-tube or embryo-sac. In the archegoniate cryptogams, on the 

 other hand, each spore gives rise, by repeated divisions, to a " sexual 

 generation " (prothallium, etc.) that intervenes between the process 

 of reduction and that of fertilization. The following account deals 

 primarily with reduction in animals, the plants being afterward con- 

 sidered. 



I. Reduction in the Female. Formation of the Polar Bodies 



As described in Chapter III., the egg arises by the division of cells 

 descended from the primordial egg-cells of the maternal organism, 

 and these may be differentiated from the somatic cells at a very early 

 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. 115), 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 oocyte (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 &gg 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 afterward, as in the lamprey-eel, the frog, 

 and Amphioxits} 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. 97, 116); and in many cases 

 the first of these divides into two as the second is formed (Fig. 89). 



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 



1 Cf. p. 189. 



