GERM CELLS IN THE ARTHROPOD A 163 



ture and significance of these substances will be dis- 

 cussed later. 



2. The Keimbahn in the Crustacea 



The keimbahn in the Crustacea is best known 

 in certain Cladocera and Copepoda. Of special 

 interest are the investigations of Grobben (1879), 

 Weismann and Ischikawa (1889), Haecker (1897), 

 Amma (1911), Kuhn (1911, 1913), and Fuchs (1913). 



Grobben (1879) studied the embryology of Moina 

 rectirostris and gives a remarkably fine account of 

 early cleavage stages, considering the early date 

 when the work was done. He figures stages showing 

 a foreign body which he considered a polar body, 

 segregated in one of the early blastomeres, the segre- 

 gation and characteristics of the primordial germ 

 cell and the first entoderm cell, and the division and 

 later history of the germ cells. His results have been, 

 in the main, confirmed by Ktihn (1911, 1913). 



Weismann and Ischikawa (1889) have contributed 

 an interesting account of the primary cellular differ- 

 entiation in the fertilized winter eggs of six species 

 of the Daphnid^, belonging to four genera. The 

 germinal vesicle in the eggs of these species casts 

 part of its chromatin contents into the cytoplasm 

 which there became organized into a "Paranucleus." 

 This paranucleus then acquired a cell body and in 

 this condition was termed the "Copulationszelle" 

 because of its future history. In two of the species 

 examined this Copulationszelle united with one of 

 the first two cleavage cells ; in the other four species 



