THE GAMETES IN PLANTS AND ANIMALS 325 



two-cell stage, consists in the separation of the large club-shaped 

 ends of the chromosomes, their exclusion from the nucleus of the 

 following resting stage, and their gradual disappearance in the 

 cytoplasm. At the same time the remaining portions of each 

 chromosome break up into a number of smaller chromosomes and 

 in following divisions of this cell similar small chromosomes appear, 

 and the nuclei of the resting stages are relatively small and poor 

 in chromatin. In the other cell, however, diminution does not 

 occur, the chromosomes retain their original form and large size, 

 and the resting nucleus is large and rich in chromatin. In the 

 second cleavage this cell gives rise to one cell which undergoes 

 diminution and one which does not, and in the third and fourth 

 cleavages also one cell remains with chromatin undiminished. In 

 the fifth cleavage the undiminished cell divides into two equal cells, 

 and these are, according to Boveri and others, the primitive germ 

 cells. Here then we can trace the line of descent of the germ cells, 

 the germ path (Keimbahn), from the first cleavage. The germ 

 path and the fates of the various cells which undergo diminution 

 are indicated in Fig. 142. 



The process of early segregation of germ cells in Ascaris has been 

 very generally regarded as constituting almost a demonstration 

 of the continuity and independence of the germ plasm, but as a 

 matter of fact it is far from being anything of the kind. In the 

 first place, while it seems fairly certain that the reproductive organs 

 of Ascaris do arise from the undiminished cell line of descent, it is 

 not known whether these cells give rise merely to the germ cells or 

 to the walls of the reproductive organs as well. In the latter case 

 the germ path of early cleavage has not resulted in the segregation 

 of germ plasm from the soma, but merely in the segregation of 

 different organs, for the walls of the reproductive organs are not 

 germ plasm. 



Moreover, the whole process is very different from what we 

 should expect in a segregation of germ plasm from the soma. If 

 the germ plasm is a distinct entity, why should it not become 

 segregated in the first division instead of in the fourth ? The first 

 four cleavages are really segregations into different cells, not simply 

 of germ plasm, but of various parts of the body, as Fig. 142 shows. 



