328 SENESCENCE AND REJUVENESCENCE 



appear at one pole of the spindle of the cell to which the granules 

 passed in the preceding division, and in each case pass into one of 

 the two daughter cells, which continues the germ path. But in 

 the cell of the fifth generation the granules appear all around the 

 mitotic figure and pass into both daughter cells, which are according 

 to Hacker the primitive germ cells. Here the germ path is charac- 

 terized, not by peculiar nuclear features, but by cytoplasmic differ- 

 entiations which are products of metaboHsm: the germ cells are 

 evidently an integral physiological part of the organism. Some- 

 what similar germ paths have been described for various other 

 Crustacea. 



The early segregation of the primitive germ cells in Sagitta has 

 been noted by several authors, and Buchner ('lo) has recently 

 discovered the beginning of the germ path in the granules resulting 

 from the degeneration of a nutritive cell taken up by the egg in 

 the ovary, again a cytoplasmic not a nuclear basis of segregation, 

 although the granules in this case may be of nuclear origin. In a 

 discussion of other cases Buchner concludes that determination of 

 the germ path in this way is of very general occurrence. 



In many insects a distinct germinal path with early segregation 

 of the primitive germ cells has been observed. Among the diptera 

 all forms carefully examined show some sort of germ path. In the 

 gnat Chironomus, for example (Hasper, 'ii), the primitive germ 

 cell is segregated in the second cleavage (Fig. 145), and in the fly 

 Miastor (Kahle, '08; Hegner, '12, '14a, '14c) the segregation of the 

 mother germ cell occurs in the third cleavage, one nucleus of this 

 cleavage becoming imbedded in a pecuhar cytoplasmic region at the 

 posterior end of the egg, and giving rise later to the germ cells, 

 while all the other nuclei undergo a process of diminution of chro- 

 matin somewhat similar to that occurring in Ascaris. 



A cytoplasmic germ-path determinant in the form of a peculiar 

 granular cytoplasmic region at the posterior pole of the egg, which 

 during cleavage becomes nucleated and separates off as the primi- 

 tive germ cells, has recently been described for several chrysomelid 

 beetles, including the potato beetle, by Hegner ('09, '11, '14a). 

 This author concludes with Boveri that the cytoplasm, not the 

 nuclei, determines which cells shall become germ cells, but this 



