120 



GENERAL CONCEPTS 



SPERMATOGENESIS 



A 



OOGENESIS 



Spermatogonia in 

 V testis and oogonia 

 ''^ in ova.ry (divide * 



meuiy times by 



mitosis. 



J 



Primat-y — — 

 Spermato cy t s. 



A sperm alo^onium 

 Orows into a 



1iir 



odary - 



beconc 

 Sperina.toc_yt£ 



FIRST 



MEIOTIC 



DIVISION 



Spsrmalids 



I 



4 



SECOND 



MEIOTIC 



DIVISION 



/ \ 



r\. 



V 



An oogonium, 

 brows into a. 



i 



m m 



Primary 

 oocyte 



First 

 polocyte 



Se-conda-ry 

 oocyte 



iO 



\ i \ 



Second 

 polocyte 



Zygote 

 Figure 6.3. Comparison of the formation of sperm and eggs. 



of sizes and shapes (Fig. 6.5). The sperm of a few animals, such as the 

 parasitic roundworm Ascaris, lack tails and crawl along by ameboid 

 motion. Crabs and lobsters have curious tailless sperm with three pointed 

 projections on the head. These hold the sperm in position on the sur- 

 face of the egg while the middle piece uncoils like a spring and pushes 

 the sperm nucleus into the egg cytoplasm, thereby accomplishing fer- 

 tilization. 



Oogenesis. The immature sex cells in the ovary are known as 

 oogonia. These undergo successive mitotic divisions to form additional 

 oogonia during development. When the individual reaches sexual ma- 

 turity, oogonia develop into large primary oocytes. These are typically 

 much larger than the corresponding primary spermatocytes and con- 

 tain yolk, which will serve as food in the event the egg is fertilized. 

 Some of the "morphogenetic substances" which subsequently regulate 

 the development of the fertilized egg are formed at this time. When it 

 has completed its growth phase the primary oocyte divides by the first 

 meiotic division (Fig. 6.3). The two daughter ceils, however, are not 

 of equal size. One, the secondary oocyte, receives essentially all of the 



