44 GERM-CELL CYCLE IN ANIMALS 



divided into segments, the chromosomes, which are 

 only haploid in number. The reduction from the 

 diploid to the haploid number is brought about by 

 the union of the chromosomes in pairs, a condition 

 called synapsis. Each of the haploid chromosomes 

 thus consists of two of the diploid chromosomes 

 and is said to be bivalent. That one of the chromo- 

 somes of each pair is of maternal origin, i.e., is a 

 descendant of a chromosome present in the egg at 

 the time of fertilization, and the other of pater- 

 nal origin, i.e., a descendant of one brought into 

 the egg by the spermatozoon, seems to be well 

 established. The final act of fertilization, therefore, 

 occurs at this point in the germ-cell cycle — an 

 act of much greater significance than that of the 

 union of the egg and spermatozoon. Furthermore, 

 there is considerable evidence that the chromo- 

 somes differ one from another and that in synapsis 

 corresponding (homologous) chromosomes unite. 

 The importance of such a union from a theoretical 

 standpoint will be discussed later. 



The nuclei now prepare for the two maturation 

 mitoses. In manv nematodes, annelids, and arthro- 

 pods these are characterized by the formation of 

 tetrads. Divisions of this sort may be illustrated 

 as in Fig. 10. The diploid number of chromosomes 

 is for convenience supposed to be four, as in the sper- 

 matogonium A. During the spermatogonial divi- 

 sions these divide as in B, so that each daughter cell 

 receives the diploid number, four. After synapsis, 

 however, each of the haploid chromosomes of the 



