26 TEXT-BOOK OF EMBRYOLOGY. 



bling of chromatin material at the subsequent fertilization. This, however, 

 seems to be but a minor object of maturation. As a matter of fact, the reduc- 

 tion of the chromatin mass is not one-half but three-quarters and even more. It 

 is also well known that the chromatin mass increases or diminishes under cer- 

 tain conditions during the life history of a cell. 



The chief significance of maturation is to be considered rather from the 

 standpoint of heredity. Modern biologists believe that the chromatin particles 

 are the bearers of the hereditary qualities of the cell. During mitosis the chro- 

 matin granules arrange themselves in a continuous thread, the spireme, which 

 differs qualitatively in different regions. The chromosomes, which are only 

 segments of the spireme, likewise differ from end to end. In ordinary mitosis 

 these chromosomes split longitudinally, half of each chromosome going to each 

 of the resulting daughter cells. This is an equational division in which the 

 chromatin material is exactly halved. 



In maturation, however, a synapsis of the chromosomes takes place, the 

 latter fusing in pairs. The chromosomes of each pair are probably separated 

 again in one of the subsequent maturation divisions, the reduction division. 

 If the chromosomes are qualitatively different, then the mature germ cells re- 

 sulting from this division will be of two different kinds, varying more or less 

 in their content of hereditary factors. Experimental evidence confirms this 

 interpretation of maturation. 



There is another interesting point to be considered. The recent work of 

 cytologists leads to the assumption that the fusion of chromosomes during syn- 

 apsis is not a matter of chance, but takes place in a very definite manner. The 

 chromosomes in the primordial germ cells seem to form a series of homologous 

 pairs the members of which fuse during synapsis. The individual pairs can 

 often be distinguished from other pairs by differences in shape or size. There 

 is much evidence to support the belief that each pair consists of one paternal 

 and one maternal chromosome, which had been brought together at the ante- 

 cedent fertilization. This seems to indicate also, as mentioned on page 7, 

 that the chromosomes retain their identity even when resolved into the chro- 

 matic reticulum of the resting nucleus. The reduction division will separate the 

 fused chromosomes, and the resulting mature germ cells will be either paternal 

 or maternal in their chromatic constitution. The maturation processes there- 

 fore produce a segregation of the paternal and maternal chromosomes. 



The cytological data described above, which support and in turn are sup- 

 ported by a great mass of experimental evidence, illustrate Mendel's "law of 

 segregation." This law is that "the units contributed by the two parents 

 separate in the germ cells without having had any influence upon each other." 

 For instance, when a mouse with gray coat color is mated with a mouse with 

 black coat color, one parent contributes a unit for gray and the other a unit 



