STUDIES ON GERM CELLS 381 



gart, '05); in the aphids (Metschnikoff, '66; Balbiani, '66-'72; 

 Witlaczil, '84; Will, '88); in the honey-bee (Petrunkewitsch, 

 '01-03); and in Forficula auricularia (Heymons, '95). 



Since the original work contained in this paper was under- 

 taken in order to determine the origin and significance of certain 

 pecular inclusions in the primordial germ cells of various animals 

 the writer has been particularly interested in any extra-nuclear 

 substances visibly different from the general cytoplasm. One 

 of the principal characteristics used for the purpose of identify- 

 ing germ cells in the embryos of animals is the presence within 

 their cytoplasm of yolk substance. Many of the authors cited 

 above noticed yolk globules in the pole-cells. For example, in 

 Chironomus, Weismann ('63) states that each pole-cell possesses 

 ''ein oder zwei Dotterkornchen;" and Metschnikoff ('66) de- 

 scribed dark yolk masses in the pole-cells of Simula and Miastor. 

 These examples indicate the general presence of yolk-like sub- 

 stances in the primordial germ cells of the Diptera, but it re- 

 mained for later more detailed investigations with finer methods 

 to determine the origin and fate of these cytoplasmic inclusions. 

 Five papers have appeared which contain information bearing 

 on these problems; (1) Ritter ('90) on Chironomus, (2) Noack 

 ('01) on Calliphora, (3) Kahle ('08) on Miastor, (4) Hasper ('11) 

 on Chironomus, and (5) Hegner ('12) on Miastor. 



Chironomus. As stated above, the pole-cells of Chironomus, 

 were first described by Weismann ('63) who, however, did not 

 recognize them as germ-cells. Grimm ('70) succeeded in tracing 

 the pole-cells in Chironomus until they became surrounded by 

 other cells, forming two germ-glands, thus confirming Metschni- 

 koff's ('66) account in Miastor. Chironomus was later studied 

 again by Weismann ('82), by Balbiani ('82, '85), by Jaworowski 

 ('82), by Ritter ('90) and by Hasper ('11). Only the work of the 

 last two needs to be considered here since that of the other writers 

 mentioned was not carried on with modern methods nor in such 

 great detail. 



Ritter ('90) used the section method and was thus able to study 

 the structure of the germ cells more carefully and to trace them 

 more accurately during embryonic development. He found 



