STUDIES ON GERM CELLS 475 



the posterior end of the abdomen is also absent (Hegner, '11a). 

 The pole-disc granules and the cytoplasm containing them are 

 moved by centrifugal force toward the heavy end of the egg and 

 the latter is proved to be quite rigid, but eggs thus treated do not 

 develop sufficiently normally to enable one to decide whether the 

 pole-disc produces germ cells in its new environment or not. 



That the germ cells of Chironomus arise from a prelocalized 

 substance was stated by Balbiani ('85) in these words, "les 

 glandes genitals des deux sexes ont une origine absolutment 

 identifique, naissant de la meme substance et au meme point de 

 I'oeuf." Later Hitter ('90) expressed the opinion that the 'Keim- 

 wulst' of Chironomus consists "aus feinkornigem Protoplasma," 

 an opinion concurred in by Hasper ('11) who terms it 'Keimbahn- 

 plasma.' The similar material in Miastor metraloas; — the 'polares 

 Plasma'— is considered a special sort of protoplasm by Kahle ('08) 

 and I can confirm this for Miastor americana. Further evidence 

 of the protoplasmic nature of the substances which become segre- 

 gated in the primordial germ cells is furnished by Boveri's ex- 

 periments on Ascaris. In 1904 this investigator concluded from 

 a study of dispermic eggs that the diminution process is controlled 

 by the cytoplasm and not by an intrinsic property of the chromo- 

 somes, and that the chromosomes of nuclei lying in the vegetative 

 cytoplasm remain intact, whereas those of nuclei imbedded in 

 the animal cytoplasm undergo diminution. This conclusion 

 has been strengthened by more recent experimental evidence 

 (Boveri, '10) both from observation on the development of 

 dispermic eggs and from a study of centrifuged eggs (fig. 19, 

 p. 378). Boveri's results furnish a remarkable confirmation 

 of the conclusions reached by the writer from a morphological 

 study -of the germ cells of Chrysomelid beetles and expressed in 

 the following words: "All the cleavage nuclei in the eggs of the 

 above named beetles are potentially alike until in their migration 

 toward the periphery they reach the 'Keimhautblastem.' Then 

 those which chance to encounter the granules of the pole-disc 

 are differentiated by their environment, i.e., the granules, into 

 germ-cells; all the other cleavage products become somatic cells." 



