310 REPRODUCTION IN ANIMALS 



Early History of Germ Cells. — Although the repro- 

 ductive elements are held within the body of an animal, 

 much like peas in a pod, this does not mean that during 

 the months or years that they are thus imprisoned they 

 lie dormant. As a matter of fact germ cells have a very 

 definite development which begins at an early age and 

 leads up to their completed form ; and this history in its 

 essential features is the same for both the sperms and 

 the eggs. 



In many animals the germ cells are recognizable in 

 the gastrula stage or even earlier, though in numerous 

 other species they are not distinguishable until a much 

 later period. These original or primitive sex cells, as 

 they are termed, as a result of repeated division (Fig. 75) 

 become very numerous, and, closely packed together, 

 may for a time cease to develop further. Sooner or later, 

 however, they enter a growth period, during which time 

 they increase in size, owing to the absorption of food 

 (yolk in the case of eggs), and become, according to the 

 sex of the parent, either sperm or egg mother cells. They 

 now undergo two final reducing or maturation divisions 

 that in three respects are of supreme importance. The 

 maturation of the sperm cells will first be described. 

 • Reducing Division. — It will be recalled that in the 

 act of fertilization two nuclei, each carrying four chromo- 

 somes, in the species chosen as an illustration, fuse and 

 the resultant cell thus contains eight. At each of the 

 succee(hng divisions each of these chromosomes splits 

 lengthwise so that every cell of the body, including the 

 germ cells up to the maturation divisions, possesses this 

 characteristic number. At the beginning of the division 

 of a sperm mother cell, eight chromosomes appear, but 

 instead of splitting they unite or conjugate in pairs, one 

 maternal with one pktemal (Fig. 76). If the chromo- 

 somes that came in with the sperm at the time of fertiliza- 

 tion be designated a, b, c, d, and those of the egg be desig- 

 nated as A, B, C, D, then the conjugation of the 



