GERM CELLS IN NEMATODES, SAGITTA 179 



granular ball (Fig. 53, A, B). This phenomenon was 

 reported by Hogue (1910) and such eggs were termed 

 "Balleier." In these eggs the two cells of the four- 

 cell stage which are adjacent to the "Ball" undergo 

 the diminution process (Fig. 53, A, AB) ; the re- 

 maining two are stem cells which give rise to the 

 germ cells (Fig. 53, A, P). Thus there are two 

 "Keimbahnen" proceeding side by side in a single 

 egg and four primordial germ cells are produced in- 

 stead of two as in normal eggs (see Fig. 51). Miss 

 Hogue's experiments with centrifugal force led her 

 to conclude that these must be an "unsichtbare 

 Polaritat" or " Protoplasmaachse " in the egg of the 

 Ascaris. Boveri agrees with this and considers 

 further that the initiation of the diminution process 

 is not determined by the chromatin but by the 

 cytoplasm of the egg. 1 



2. THE KEIMBAHN IN SAGITTA 



Sagitta has proved to be of considerable impor- 

 tance to those interested in the keimbahn of animals. 

 Hertwig (1880) figures the four primitive germ cells 

 in the gastrula and later stages, proving that these 

 cells are early set aside in embryonic development. 

 Recently the work of Elpatiewsky (1909, 1910) has 



1 He states that, " Was aber auch hier durch weitere Untersuchungen 

 noch erreicht werden mag, Eines halte ich fiir sicher, dass sich alles, was 

 iiber die Wertigkeit der primaren Blastomeren bei abnormer Furchung 

 ermittelt worden ist, durch die Annahme sehr einfacher Plasmadifferenzen 

 erklaren lasst, wogegen die Hypothese einer differenzierenden Wirkung 

 des Kerns in jeder Form auf imiiberwindliche Schwierigkeiten stosst " 

 (p. 206). 



