MATURATION. 



21 



filament which winds around the axial filament of the middle piece. Mean- 

 while the axial filament has been growing in length and part of it projects be- 

 yond the limits of the cell. The cytoplasm remaining attached to the anterior 

 part of the filament surrounds it as the sheath of the middle piece. In Mam- 

 mals there appears to be more cytoplasm than is needed for the formation of 

 the sheath of the middle piece, and a large part of it degenerates and is cast 

 aside. The sheath which surrounds the main part of the axial filament appears 

 in some cases at any rate to develop from the filament itself. The galea capitis 

 or delicate film of cytoplasm which covers the head is undoubtedly a remnant 

 of the cytoplasm of the spermatid. 



The developing spermatozoa lie with their heads directed toward the base- 

 ment membrane, and attached, probably for purposes of nutrition, to the free 

 ends of the Sertoli cells (Fig. 9). Their tails often extend out into the lumen 

 of the tubule. When fully developed they become detached from the Sertoli 

 cells and lie free in the lumen of the tubule. 



MATURATION OF THE OVUM. 



The female germ cell, before it is fertilized, goes through a process of matu- 

 ration similar to that of the male germ cell. The result is essentially the same 



m.pn. 



FIG. 12. From sections of ova of the mouse, showing three stages in the maturation process. 



A, Ovum showing prophase of maturation division. /, fat; z.p., zona pellucida. 



B, Ovum showing maturation spindle with chromatin segments undivided. 



C, Ovum showing diaster stage of maturation division, formation of ist polar body (p.b.), and sperm 



nucleus (male pronucleus, m.pn.) just after its entrance. Sobotta. 



the mature ovum contains a reduced number of chromosomes. There is this 

 difference, however, that while the chromatin elements are distributed equally 

 during the reduction divisions, one cell only retains practically all the cytoplasm 

 and deutoplasm present in the primary oocyte. This cell becomes the func- 

 tional ovum while the other cells are pinched off as minute bodies, containing 

 but little of the cytoplasm, which are known as polar bodies and eventually 

 degenerate and die (Figs. 12 and 13). 



The early maturation stages of the female sex cell are very similar to those 



