THE GERM CELLS 



21 



child. Hensen estimates that a normal human female may develop in each ovary 

 200 ripe ova. Most of the young ova, which may number 50,000, degenerate and 

 never reach maturity. At ovulation but one ovum is normally ripened and dis- 

 charged from the ovary. Several 

 ova, however, may be produced in 



Such 



Head 



y GaJea 



Neck 



Body , 



end ring 



Main segment 

 of Tail* 



-intend of Knob 

 -Post end of Knob 



.Spiral fibers 



^Sheath of axial 

 thread 



a single follicle in rare cases, 

 multiple follicles have been ob- 

 served in human ovaries and are of 

 frequent occurrence in the ovary of 

 the monkey. Fig. 6 shows such a 

 follicle containing five immature ova. 



The Spermatozoon. The male 

 cell or spermatozoon is a minute cell 

 0.05 mm. long, specialized for active 

 movement. Because of their active 

 movements, spermatozoa were, when 

 first discovered, regarded as para- 

 sites living in the seminal fluid. The 

 sperm cell is composed of a flattened 

 head, indistinct neck piece, and thread- 

 like tail (Fig. 7). 



The head is about 5 micra in 

 length. It appears oval in side 

 view, pear-shaped in profile. When 

 stained, the anterior two-thirds of 

 the head may be seen to form a cap, 

 and the sharp border of this cap is 

 the perforatorium by means of which 

 the spermatozoon penetrates the 

 ovum. The head contains the nu- 

 clear elements of the sperm cell. 

 The neck is said to be disc-shaped 

 and to contain the centrosomes as 

 the anterior and posterior centro- 



some bodies. The tail is divided into a short connecting piece, a flagellum which 

 forms about four-fifths of the length of the sperm cell and a short end-piece 

 (Fig. 7). The connecting piece is marked off from the flagellum by the annulus. 



~/Jxial thread 

 -Capsule 



Terminal filament 



FIG. 7. Diagram of a human spermato- 

 zoon, highly magnified, in side view (Meves, 

 Bonnet). 



