The Development of the Vertebrate Embryo 49 



1. The head. This portion is taken up almost completely by the 

 sperm nucleus. 



2. The midpiece. This part contains material which will be put to 

 use after the sperm penetrates the egg. It aids in the fusion of the egg and 

 sperm nuclei and in the cleavage of the fertiHzed egg into the first two 

 daughter cells. 



3. The tail. This can be long or short, flexible or stiff. It may vibrate, 

 whip, or rotate in a screw-like motion depending on the species. It is the 

 locomotor organelle. 



Fig. 24. Maturation of the sperm. 



The ancestors of the sperm start out as ordinary diploid individuals. 

 The germ cells, as they are called (both in males and females), arise early 

 in the development of the embryo. They appear to be irreplaceable, be- 

 cause when embryos are treated in a manner that destroys or removes ( by 

 ultraviolet irradiation or surgery) the germ cells, the resultant adult or- 

 ganisms are sterile. 



Gameto genesis, the process by 

 which the diploid germ cells give 

 rise to haploid sperm or eggs, is 

 properly within the domains of cy- 

 tology and genetics but will be re- 

 viewed briefly here. Figures 24 and 

 25 are schematic summaries of the 

 maturation of male and female 

 gametes. For simplicity, we imagine 

 that the diploid nucleus contains 

 only two pairs of chromosomes. 

 Each chromosome duplicates, yield- 

 ing a quartet or tetrad of chroma- 

 tids. The cell itself then divides into 

 two, each with a pair of sister chro- 

 matids. This is meiosis I. These cells 

 in turn divide without chromosomal 

 duphcation to yield four (meiosis 

 11) so that the daughter nuclei end 

 up with two chromosomes rather 

 than the original four and are there- 

 fore haploid. In the case of sperm, 

 the daughter cells are changed from 

 ordinary-looking cells into func- 

 tional gametes after meiosis II. 



