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, 7.e., is a 
descendant of a chromosome present in the egg at 
the time of fertilization, and the other of pater- 
nal origin, 7.e., a descendant of one brought into 
the egg by the spermatozoén, 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 spermatozoén. 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 many 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 
