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 Mvastor by Kahle (1908) and 
Hegner (1912, 1914a), and in Compsilura by Hegner 
(19142). 
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) Muastor (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. 
