44 PHYSICAL BASIS OF HEREDITY 



chromosome splitting lengthwise into equivalent halves as 

 in ordinary cell-division. 



The interpretation of these two divisions that occur 

 in the egg and in the sperm-cell has been the subject of 

 much speculation. It is apparent that the process reduces 

 the number of chromosomes by half, and that the whole 

 number is regained by fertilization. It is sometimes said 

 that the "purpose" of this division is to keep the number 

 of chromosomes constant, for, if not reduced, they would 

 increase in number with each fertilization. 



The '^reason" for the other, the second, division is 

 acknowledged to be obscure. For present purposes it is 

 futile to speculate concerning these two divisions, but it 

 should be pointed out here that the genetic evidence is in 

 full accord with the interpretation of these two divisions 

 that is generally accepted to-day by cytologists, i.e., that 

 one of the divisions separates the conjugating pair, and 

 that the other represents a longitudinal division within a 

 paternal and within a maternal chromosome of each pair. 



If w^e follow the history of the germ-cells further back 

 before the maturation divisions, we find that between the 

 stage when the half number of chromosomes reappears 

 (tetrads) and the stage at which the full number was 

 present, there is a very obscure period in the history of 

 the germ-cells. This period has been studied chiefly in 

 the male. Only a few types have been found favorable 

 for the study of this period. One of the most favorable 

 ones is a marine annelid, Tomopteris, studied by the 

 Schreiners. The early division of the germ-cells (the 

 spermatogonia) of Tomopteris, when the full number 

 of chromosomes is present, is show^n in Fig. 18, a-g. 

 The division is like that of all the other cells of the body. 

 The chromosomes appear as thick bent threads that split 

 lengthwise (Fig. 18,a, b). The nuclear wall disappears and 

 a spindle appears near the group of split chromosomes 

 (Fig. 18, c). As the poles of the spindle move apart the 

 chromosomes become arranged at the equator of the spin- 



