446 ROBERT W. HEGNER 



Miss Hogue's experiments with centrifuged force led her to con- 

 clude that there must be an 'unsichtbare Polaritat' or 'Pro- 

 toplasmaachse' 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. He states that 



Was aber auch hier durch weitere Untersuchungen noch erreicht 

 werden mag, Eines halte ich fiir sicher, dass sich alles, was liber die 

 Wertigkeit der primaren Blastomeren bei abnormer Furchung ermit- 

 telt worden ist, durch die Annahme sehr einfacher Plasmadifferenzen 

 erklaren lasst, wogegen die Hypothese einen differenz'erendenWirkung 

 des Kerns in jeder Form auf uniibermndliche Schwierigkeiten stosst 

 (p. 206). 



3. THE KEIMBAHN IN SAGITTA 



Sagitta has proved to be of considerable importance to those 

 interested in the Keimbahn of animals. Hertwig ('80) figures 

 the four primitive germ cells in the gastrula and later stages, 

 proving that these cells are early set aside in embryonic develop- 

 ment. Recently the work of Elpatiewsky ('09, '10) has given 

 Sagitta a new importance, since this writer has found within the 

 fertilized egg a cytoplasmic inclusion which is intimately associ- 

 ated with the segregation of the germ cells. The presence of 

 this inclusion has been confirmed by Buchner ('10a, '10b) and 

 Stevens ('10) and several ideas have been expressed regarding 

 its origin, fate and significance. 



Elpatiewsky ('09) found in Sagitta, at the time when the male 

 and female nuclei were lying side by side in the middle of the egg, 

 a body situated near the periphery at the vegetative pole (fig. 22, 

 B, x). This body, which he called the 'besondere Korper,' 

 consists at first of 'grobkornigen' plasma which stains hke chro- 

 matin but not so intensely; later it condenses into a round homo- 

 geneous body with a sharp contour. During the first five cleav- 

 age divisions the 'besondere Korper' is always confined to a 

 single cell. At the completion of the fifth cleavage (32-cell 

 stage), the blastomere containing this cytoplasmic inclusion is 

 recognizable as the first 'Urgeschlechtszelle' (fig. 22, C, G), and 



