292 EVOLUTION AND GENETICS 



He has also made comparisons of many cells in which their arrange- 

 ment appeared to be constant (Fig. 166). 



Characteristics of Chromosomes. While the inner organization 

 of the chromosomes is a subject for cytological studies of the 

 utmost refinement, the bodies themselves are more readily ob- 

 served and exhibit several phenomena of interest. 



1. Chromosomes appear in the cells of a given species in the 

 same number and form, with the exception of a common discrep- 

 ancy between the sexes (Fig. 168). 



2. Within the chromosome complex of a given species there 

 are chromosomes of different sizes and shapes. In species which 

 show chromosomal differences sufficient for accurate observation 

 it is evident that these characteristics are constant for different 

 individuals. 



3. In the cells of the body the chromosomes are duplicated, i.e., 

 there are two of each kind, with the sole exception of those associ- 

 ated with sex. 



The Chromosomes in Reproduction. No single phase of the 

 behaviour of chromosomes is as significant as the series of changes 

 that take place during the formation and union of the germ cells. 

 In the higher organisms these are of two distinct types, the female 

 gamete or ovum and the male gamete or spermatozoon of animals. 

 Both are fundamentally similar in that they possess one half of 

 the total number of chromosomes characteristic of the species, 

 usually called the haploid number, and in the process of gameto- 

 genesis by which they are produced. By the union of two germ 

 cells the full or diploid number of chromosomes is restored. 



Gametogenesis. The body of an organism during development 

 contains many primordial germ cells which are directly descended 

 from the original fertilized ovum with which its development 

 began. These primordial germ cells multiply by mitosis, and by 

 the completion of embryonic life have produced many other cells 

 which lie in the gonads. Those of the female are called oogonia 

 and those of the male spermatogonia (Fig. 169A). After a period 

 of growth they become the primary oocytes and primary spermato- 

 cytes respectively, and in this stage a significant step occurs in the 

 behaviour of their chromosomes, known as synapsis (Fig. 169C). 



Synapsis. In brief, synapsis consists of a pairing of similar 

 chromosomes from the diploid complex. It is attended by a 

 considerable degree of complexity in some animals, but in its 



