ONTOGENESIS 153 



thing apart from the soma-plasm, its cells, like soma- 

 cells, go through the stages of progressive special- 

 ization, characteristic of individual development 

 (ontogeny). Of the physico-chemical changes in- 

 volved in- this specialization we have no inkling. 

 The external and visible changes incident to its 

 conclusion have been carefully studied, and to them 

 is usually applied the term maturation. In animals, 

 the process is practically the same, up to the final 

 stage, in both sperm and ova. It may be roughly 

 divided into three successive periods : a period of 

 multiplication of individual pro-gametes (apparently 

 without differentiation), followed by a period of 

 growth, very much more extensive in the ovum 

 than in the sperm, and lastly, a period of differen- 

 tiation which is known as reduction. In many 

 species the eggs are shed from the parental body 

 at the conclusion of the second period. But before 

 zygosis can occur, reduction must always take place. 

 In this remarkable phenomenon the egg or sperm 

 divides twice in rapid succession, by mitosis, a 

 mitosis, however, which differs markedly from all 

 others known, in two particulars. In the first place, 

 when the chromatin skein begins to condense and 

 resolve itself into chromosomes, instead of the usual 

 number of chromosomes there appear in the spindle 

 of the maturing gamete but half as many as the 

 normal number. These are often arranged in 

 groups of four called tetrads. The elements of the 



1 It is believed that the tetrad arises by a fusion of chromosomes in 

 pairs (synapsis) and the subsequent splitting of the fused chromosomes, 



