170 



THE DEVELOPMENT OF THE GAMETES OR SEX CELLS 



b. Physiological Ripening of the Female Gamete 



The physiological maturing of the oocyte is linked to factors which influence 

 the developing egg at about the time the maturation divisions occur. Sea-urchin 

 sperm may penetrate the egg before the maturation divisions occur (Chap. 5). 

 However, development does not take place in such instances. On the other 

 hand, sperm entrance after both maturation divisions are completed initiates 

 normal development. In the protochordate, Styela, marked cortical changes 

 transpire at about the time the egg leaves the ovary, and as it reaches the 

 sea water, the germinal vesicle begins to break down. The oocyte becomes 

 fertilizable at about this time. In Amphioxus, although the first polar body is 

 given off within the adult body, the egg apparently is not fertilizable until it 

 reaches the external salt-water environment. The secondary oocyte of the 

 frog presumably must remain within the uterus for a time to ripen in order 

 that ensuing development may be normal. These and other instances suggest 

 that physiological changes — changes which are imperative for the normal 

 development of the egg — are effected at about the time that the maturation 

 divisions occur. 



D. Summary of Egg and Sperm Development 



From the foregoing it may be seen that the development of the gametes in 

 either sex involves a process of maturation. This maturation entails changes 

 in the structure and constitution of the nucleus and cytoplasm, and, further, 

 a functional or physiological ripening must occur. The comparative maturation 

 events in the egg and sperm may be summarized as follows: 



Egg {in Oogenesis) 



Sperm (in Spermatogenesis) 



Nuclear maturation 



a. Homologous chromosomes synapse 

 and undergo profound changes dur- 

 ing which parts of homologous chro- 

 mosomes may be interchanged; ulti- 

 mately, the chromosome number is 

 reduced to the haploid number 



b. Nucleus enlarges, and contained nu- 

 clear fluid increases greatly; ulti- 

 mately the nuclear fluid is contrib- 

 uted to cytoplasm upon germinal 

 vesicle break down 



c. Nuclear maturation occurs simulta- 

 neously with cytoplasmic differenti- 

 ation 



1. Nuclear maturation 



a. (Similar to the female) 



b. Nucleus remains relatively small and 

 enlargement is slight; nuclear fluid 

 small in amount; during spermio- 

 genesis the nucleus may contract into 

 a compact mass; considerable elon- 

 gation of nucleus occurs in many 

 species 



c. Nuclear maturation occurs before 

 spermiogenesis or cytoplasmic differ- 

 entiation 



