470 ROBERT W. HEGNER 



number of the Keimbahn-determinants which have been described 

 are supposed to consist of nutritive substances. Some of the 

 earhest investigators were aware of the yolk content of the pri- 

 mordial germ cells. For example, in Chironomus Weismann 

 ('63) found four oval nuclei lying in the 'Keimhautblastem' 

 at the posterior end of the egg; each of these, he says, "besassen 

 einen Kreisrunden, klaren, etwas rothlich schimmernden Kern, 

 und in einigen Lagen ausserdem noch ein oder zwei Dotterkorn- 

 chen." These are the Tolzellen.' In another dipteron (Simula 

 sp.) Metschnikoff (1866) records four or five pole-cells which 

 "bestehen ausser einem Kerne noch aus einer die feinsten Dotter- 

 kornchen enthaltenden Zellsubstanz." The same author ('66) 

 also states that when the pseudovum in the paedogenetic larva 

 of Miastor contains twelve to fifteen nuclei, "Man bemerkt 

 zunachst, dass der am spitzen Pole des Pseudovums hegende 

 Keimkern von einer dicken dunkeln Dottermasse scharger 

 umgeben wird und mit dieser zusammen bald in eine besondere, 

 0.017 mm. grosse, membranlose Zelle sich abschniirt." This 

 gives rise to the pole-cells. 



In certain Daphnidae, Weismann and Ischikawa ('89) describe 

 a Taracopulationszelle' which is derived from the contents of the 

 germinal vesicle (p. 433); but the recent work of Kuhn ('11, '13) 

 renders it probable that this body is nothing but the remains of 

 a nurse cell. The 'Dotterplatte' discovered by Noack ('01) 

 at the posterior end of the egg of Calliphora (fig. 2) is considered 

 by this investigator to consist of yolk elements. In previous 

 communications (Hegner, '08, '09a, 'lib) the writer has discussed 

 the probability that the pole-disc in Chrysomelid eggs consists 

 of nutritive material, and Wieman ('10a) also has offered argu- 

 ments for this view. 



Klihn ('11, '13) has presented what appears to be certain 

 evidence that the Keimbahn-determinants in the egg of the Clado- 

 ceron, Polyphemus pediculus, arise from one or more nurse cells. 

 The granules segregated in certain cleavage cells of Neritina 

 (Blochmann, '82) Asplanchna (Jennings, '96), Lepas (Bigelow, 

 '02), Siphonaria (Fujitas, '04), and Physa (Wierzejski, '05) may 

 be of a nutritive nature and these cells- may be the stem cells 



