30 



PRINCIPLES OF EMBRYOLOGY 



I. Spermatogenesis 



In the development of sperm, the two meiotic divisions occur fairly 

 early in the history of the spermatocyte, usually with only a short interval 

 between them. Since there are two maturation divisions, and each division 

 gives rise to two similar daughter cells, one spermatocyte which starts the 

 process will eventually form four sperm. Usually they separate from one 

 another, but in some species they remain together as a group (Fig. 2.1). 



Spermatogonia 

 2n 



Spe^mQ^■ocy^es 



ReducHon Divisions 



ton 



SPERMATIDS 



PERIOD OF 

 DIFFERENTIATION 



SPERM 



! I I I 



Oogonia 2n 



Oocyt'e 



PERIOD OF 

 DIFFERENTIATION 



OVUM 



Reduchon Divisions 

 to n 



Formahon oF Polar Bodies 



Figure 2.1 

 Diagram of the formation of sperm (on left) and eggs (on right). 



At the time when the maturation divisions occur, the spermatocytes 

 are fairly normal-looking cells; the main differentiation by which they 

 become transformed into sperm occurs in the haploid daughter cells. In 

 a few groups, the fully differentiated sperm are amoeboid (e.g. some 

 Crustacea), but in most animals they are built on roughly the same plan, 

 consisting of three main parts; a head containing the nucleus, a middle- 

 piece containing one or more centrosomes, and a flagellar tail which 

 serves as an organ of motion. (For a comparative account of sperm 



