380 ROBERT W. HEGNER 



2. DIPTERA 



A. Historical account 



The segregation of the germ cells in the early embryonic 

 stages of animal development was first discovered in certain 

 Dipterous insects. In 1862 Robin described, in the nearly 

 transparent eggs of Tipulides culiciformes the appearance of 

 four to eight buds at one pole just previous to the formation of 

 the blastoderm. He called these buds 'globules polaires' and 

 thought that they were protruded at the anterior end. Weis- 

 mann ('63) likewise discovered bud-like protrusions at a cor- 

 responding stage in the development of the egg of Chii'onomus 

 nigroviridis and Musca vomitoria. He corrected Robin regard- 

 ing their orientation by proving that they arise at the posterior 

 end and not at the anterior end of the egg. Because of their 

 position he applied to them the term 'Polzellen/ a term that has 

 persisted until the present time. Weismann was unable to fol- 

 low the history of the pole cells and so did not succeed in deter- 

 mining their true significance. 



Metschnikoff ('65, '66) and Leuckart ('65) were the first to 

 announce that the pole cells are really primordial germ cells, and 

 the first to trace them from their initial appearance until they 

 entered into the constitution of the definitive germ glands. 

 Their results, obtained from the study of the eggs of Miastor 

 and Simula, were confirmed by Grimm ('70) and Balbiani 

 ('82, '85) in Chironomus. 



Pole cells have also been described among the Diptera, in 

 Musca by Kowalevsky ('86), Voeltzkow ('89), and Escherich 

 ('00) ; in Calliphora by Graber ('89), and Noack ('01) ; in Chirono- 

 mus by Ritter ('90), and Hasper ('11); in Lucilia by Escherich 

 ('00); and in Miastor by Kahle ('08) and Hegner ('12). 



Among insects belonging to other orders typical pole cells 

 have been found only in parasitic Hymenoptera (Silvestri, '08), 

 and in Chrysomelid beetles (Lecaillon, '98; Hegner, '08, '09; 

 Wieman, '10a) although germ cells have been described at an 

 early stage in the development of the butterflies, Euvanessa 

 antiopa (Woodworth, '89) and Endromis versicolora (Schwan- 



