The Development of the Vertebrate Embryo 51 



Formation 

 o-f te+rods 



/ \ 



The maturation of the egg, 

 hke that of the sperm, proceeds via 

 two meiotic divisions from a small 

 diploid parent cell (Fig. 25). The 

 parent cell chromosomes duplicate, 

 and the cell then grows to enormous 

 size. The proteins, fats, nucleic 

 acids, and carbohydrates that com- 

 prise the yolk are synthesized within 

 the egg or are received from 

 without (contributed by so-called 

 "nurse" cells). When the final size 

 is attained, the egg nucleus goes 

 through meiosis I and two cells ap- 

 pear. The cell division is very un- 

 equal, however, and the lucky nu- 

 cleus finds itself with most of the 

 cytoplasm and yolk, the unlucky 

 nucleus with very little. Meiosis II 

 follows and now four haploid cells 

 are present. Again, because of a 

 very unequal division, one of the 

 daughter cells contains practically 

 all the yolk and cytoplasm. This, 

 then, is the mature egg, and its 

 three tiny sisters (called polar 

 bodies ) shortly degenerate and dis- 

 appear. Figure 24 is a schematic 

 summary of the process. 



In most cases, developing eggs 

 reach stage C and then lie dormant 

 until fertilization, at which time the process is completed and the sur- 

 viving female gamete nucleus unites with the sperm nucleus. In a few 

 cases the egg is arrested at stage B and very few reach stage D before 

 fertilization. 



Fig. 25. Maturation of the egg. 



THE LIFE SPAN OF GAMETES 



A mature egg must be fertilized within a short time, if it is to be 

 fertilized at all. Thus, human eggs generally remain functional for about 

 12-24 hours after their release from the ovary. The eggs of sea urchins, 

 which are shed into the water, last for 48 hours, but the eggs of most 

 invertebrates as well as fish and amphibia must be fertilized within a few 

 minutes of being shed. The life spans of other animal eggs are of the same 

 order of magnitude as those of human eggs. 



