MATURATION. 21 



the chromatin 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 chromo- 

 some 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 

 resulting 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 

 synapsis is not a matter of chance, but takes place in a definite manner. 

 The chromosomes in the primordial germ cells seem to form a series of homol- 

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

 pairs can often be distinguished from other pairs by differences in shape 01 

 size. There is much evidence to support the belief that each pair consists oi 

 one paternal and one maternal chromosome, which had been brought to- 

 gether at the antecedent fertilization. This seems to indicate also that the 

 chromosomes retain their identity even when resolved into the chromatic 

 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 

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

 of segregation. This law is that the units contributed by two parents sepa- 

 rate 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 

 for black. These units will separate during the maturation of the germ 

 cells, and the resulting spermatozoa and ova will again recover the pure 

 paternal or maternal units. 



Sex Determination. 



In the great bulk of cytological and experimental studies of recent years 

 there is abundant evidence for the belief that certain chromosomes play an 

 important part in the determination of sex. In the grasshopper (Steno- 



