S^(B) 



Fig. 84. The Germ-track in the Threadworm 

 AscARis Megalocephala Univalens 



In the two-cell stage, A, the two chromosomes in one of the cells, P^, 

 are intact, whereas the chromosomes in the other cell, Si break up so 

 that when the cell division leading to the four-cell stage, B, is under 

 way a portion of the chromosome material is left outside the spindle, 

 Sj. In the four-cell stage, C, the broken-off ends of the chromosomes 

 of the cell Si are left outside the nucleus. Si (A) and (B), whereas 

 the chromosomes of the other two cells, derived from Pi, are intact, So 

 and Po. In the next division, D, the chromosomes of P2 remain intact, 

 but those of S2 again break up. In later stages this process of frag- 

 mentation is continued, £, and the one cell, P4, that retains the 

 original chromosome organization becomes the mother-cell from which 

 the gonads are developed. After Boveri, in Agar's Cytology, copy- 

 right by Macmillan & Company, Ltd.; from Babcock and Clausen, 

 Genetics in Relation to Agriculture, published by McGraw-Hill Book 

 Company. 



