HISTOLOGY AND CYTOLOGY 



provided at its anterior end with a covering, or cap, derived from the 

 centrosphere. This cap serves as an 

 organ for perforating the egg, and is 

 differently shaped in various species. 

 The neck is a very small inconspicuous 

 portion, but important, since it contains 

 granules derived from the centrosome of 

 the spermatid, and from which the 

 centrosomes of the fertilised ovum are 

 presumed to be derived. The tail con- 

 sists of an axial filament surrounded for 

 the greater part of its length by a cyto- 

 plasmic sheath, but three distinct parts 

 can be recognised in it. Firstly, next to 

 the neck is a moderate-sized connecting 

 piece, also containing granules derived 

 from the centrosome and other struc- 

 tures ; secondly comes a long main piece 

 with just the axis and its sheath; and 

 finally an end piece, consisting of the 

 axial filament, only without its sheath. 



The process of egg formation, 

 or obgenesis, in the female, while it 

 differs from spermatogenesis in certain 

 points, is nevertheless fundamentally the 

 same as far as the divisions are con- 

 cerned. It has not been worked out in 

 so many forms as has spermatogenesis, 

 since the ovum, being loaded with yolk 

 in the lower forms, and being inaccessible 

 in the higher animals, presents greater 

 technical difficulties in investigation. 

 Where it has been studied it has been 

 found to correspond with sperm forma- 

 tion in nuclear detail. 



The primitive germ cells undergo 

 mitotic divisions until they are quite 

 numerous, and then follows a period of 

 growth at the erM of which they are 

 oogonia. Each oogonium divides to 

 produce the primary obcytes, or ovarian 

 ova, which are provided with the diploid 

 number of chromosomes by a mitotic 

 division . These then grow until in most forms they are considerably 



ABC 



FIG. 125. Spermatozoa. 

 From Kellicott. 



A, B, two views of a human 

 sperm cell. After Retzius. X 

 2000. C, diagram of the structure 

 of a generalised type of flagellate 

 spermatozoon. After Meves. 



a., annulus ; ac, anterior cen- 

 trosome ; af, axial filament ; c., 

 cenlrosomes (end knobs) ; e., pro- 

 toplasmic envelope ; h., head ; 

 m., middle piece ; mi., mitochon- 

 dria ; n., nucleus ; ne., neck ; p., 

 perforatorium (acrosome) ; PC, 

 posterior centrosome ; s., spiral 

 filament ; t., tail piece ; tf, ter- 

 minal filament. 



