208 



PRINCIPLKS OF ANIMAL BIOLOdY 



are separated off and cast out into the cytoplasm (Fig. 161), leaving only 

 the middle portion of the chromosomes to participate in mitosis. All of 

 the germ cells of the future worm are derived from that one of the first 

 two cells which retains all of its chromatin. Not all of the cells, however, 

 which descend from that cell are germ cells, for at each of the next four 

 or five mitoses some of the cells repeat the fragmentation of their chromo- 



FiG. 161. — Early cleavage of the egg of Ascaris, showing the origin of the germ cells. 

 A, two-cell stage, with the cells preparing for another division; the chromosomes of one cell 

 remain intact, those in the other cell become fragmented; B, the same cells later, showing 

 the ends of the chromosomes in the lower cell being eliminated into the cytoplasm; C, 

 the four cells resulting from the preceding division; the lower cells contain masses of elim- 

 inated chromatin, and their nuclei are smaller; D, division of the preceding four cells, show- 

 ing a repetition of the fragmentation of the chromosomes in one of the upper cells; s, 

 stem cell, from which germ cells are derived; c, chromatin eliminated into the cytoplasm. 

 {From Wilson, after Boveri. Courtesy of Macmillan Co.) 



somes and thereby give rise to somatic cells. After about the sixth 

 division the elimination of chromatin ceases, and the cells that have kept 

 their entire chromosomes produce only germ cells. 



In insects also it has been found that the germ cells are recognizable 

 at an early stage in cleavage, either by their size or by certain granules 

 or bodies that they contain (Fig. 162). When these cells are destroyed 

 the embryo may go on and develop, but contains no reproductive cells. 



