MATURATION OF OVUM 



13 



enters the egg before the second polar body is extruded. The final phases of 

 maturation then proceed concurrently with the initial phases of fertilisation. 



While there is a general correspondence or homology between the different 

 cell-generations in spermatogenesis and oogenesis (fig. '22), it must be noted at 



3\ 



i. 



* 



FlG. 21. FORMATION OF SECOND POLAR BODY OF THE MOUSE. (Sobotta.) 



I. First polar body (a) and its nucleus (6), with second polar spindle. 

 II. Second polar body, with remains of the spindle and cell-plate, x 1200 diameters 



this point that there are two differences between the two series of cells. First, 

 after the second division is over the egg undergoes no further modification, while 

 the spermatid becomes converted, by a complicated process of cytohistogenesis, 

 into the functional spermatozoon ; secondly, all four spermatids become 



Spermatogonium 



Oogonium 



Proliferation 

 period 



Growth 

 period 



Maturation 

 period 



1234 1284 



FIG. 22. SCHEME OF SPERMATOGENESIS AND OOGENESIS. (After Boveri.) 



spermatozoa, while only one of the products of division of the original oocyte 

 becomes a functional ovum. 



From the foregoing descriptions it will be clear that in the dimorphism of the sex-cells we 

 have an instance, of a marked kind, of division of labour. In the most primitive forms among 

 the protozoa conjugation of cellular individuals occurs, but they are indifferent and equivalent 

 individuals ; while, on the other hand, in many of the higher Protozoa new conditions of life are 

 established, the sessile habit, for instance, which necessitates that one of the pair should have a 

 degree of mobility. Thus we have micro- and macro-gametes. There are instances of colonial 



