THE GAMETES IN PLANTS AND ANIMALS 353 



are highly specialized and that this specialization has been in the 

 direction of a chemical simplification, at least during the later 

 stages. Apparently the proteid constituents may undergo more 

 or less breakdown during spermatogenesis. According to Burian, 

 this process of breakdown of the proteid constituents may differ in 

 degree in different spermatozoa. So far as our knowledge goes, 

 the spermatozoa of vertebrates, except the fishes, contain typical 

 proteids as constituents of their nucleoproteids, while in the fishes 

 these are replaced by the simpler histones or the still simpler 

 protamines, and in some cases the histones are formed during 

 spermatogenesis, the protamines in the fully developed sperma- 

 tozoa. The nucleoproteids of the nuclei of other cells of the body 

 sometimes contain typical proteids, sometimes histones, in com- 

 bination with the nucleic acid, but the process of proteid breakdown 

 does not go as far as the formation of protamines. From this point 

 of view, the differentiation of spermatozoa is apparently not funda- 

 mentally different from that of other cells, but some spermatozoa 

 seem to be more highly specialized than other cells. 



As regards the eggs, it is evident that, at least in those cases 

 where they contain yolk, a progressive change in chemical consti- 

 tution of the whole cell must occur during the course of differentia- 

 tion: the most striking feature of this change is the increase in 

 lipoids, which form an important constituent of the yolk. Con- 

 cerning changes in chemical constitution of the egg nucleus we know 

 practically nothing. 



THE SIGNIFICANCE OF MATURATION 



At some point in the life history between successive generations 

 of gametes the process known as maturation occurs. In most 

 cases, both in animals and in plants, the process of maturation 

 consists of two nuclear and cell divisions during which the number 

 of chromosomes in the nucleus is decreased one-half more or less 

 (haploid number). In fertilization the normal or diploid number 

 is restored by the union of the two gametes each with the haploid 

 number. In spite of years of investigation and discussion, cytolo- 

 gists appear to be almost as far as ever from an agreement as to 

 what really occurs in the maturation divisions; indeed, it is still a. 



