GERM CELLS IN THE ARTHROPODA 107 



usual, certain species or groups of species have 

 proven more favorable than others for germ-cell 

 studies, especially those belonging to the orders 

 DiPTERA, CoLEOPTERA, and Hymenoptera. 



DiPTERA. Robin, in 1862, described what he 

 called "globules polaries" at one end of the nearly 

 transparent eggs of the crane fly, Tipulides culici- 

 formes, and the following year Weismann (1863) re- 

 ported the formation of similar cells, the "Pol- 

 zellen" at the posterior end of the eggs of the midge, 

 Chironomus nigroviridis, and the blow fly, Calliphora 

 (Musca) vomitoria. It remained for Leuckart (1865) 

 and Metchnikoff (1865, 1866), however, to identify 

 the pole cells (in Miastor) as primordial germ cells ; 

 their results were confirmed for Chironomus by 

 Grimm (1870) and Balbiani (1882, 1885). 



Pole cells have also been described among the 

 DiPTERA, in Musca by Kowalevsky (1886), Voeltz- 

 kow (1889), and Escherich (1900) ; in Calliphora 

 by Graber (1889) and Noack (1901) ; in Chironomus 

 by Ritter (1890) and Hasper (1911) ; in Lucilia by 

 Escherich (1900) ; in Miastor by Kahle (1908) and 

 Hegner (1912, 1914a), and in Compsilura by Hegner 

 (1914a). 



Four genera of flies will serve to illustrate the 

 methods of germ-cell segregation in this order : (1) 

 Chironomus (Ritter, 1890; Hasper, 1911), (2) Cal- 

 liphora (Noack, 1901), (3) Miastor (Kahle, 1908; 

 Hegner, 1912, 1914a), and (4) Compsilura (Hegner, 

 1914a). Since Miastor has been discussed in detail 

 in Chapter III it will be only briefly referred to here. 



