400 ROBERT W. HEGNER 



3. COLEOPTERA 



A . Historical account 



The primordial germ cells of beetles have not received as much 

 attention from investigators as have those of the Diptera, prob- 

 ably because they are not so conspicuous. The germ glands have 

 been described in the embryo of Hydrophilus piceus by Heider 

 ('89), in Leptinotarsa (Doryphora) decemlineata by Wheeler 

 ('89), in Melolontha vulgaris by Voeltzkow ('89b), in Hydrophilus 

 piceus, Melolontha vulgaris, and Lina tremulae by Graber ('91), 

 in a number of Chrysomelid beetles by Lecaillon ('98), and by 

 Friederichs ('06), in Tenebrio molitor by Saling ('07), and in 

 Chrysomelid beetles by Hegner ('08, '09, '09b, '11a, 'lib) and 

 Wieman (10a', 10b', '10c). Of these only the work of MTheeler, 

 Lecaillon, Hegner, and Wieman needs to be considered. Al- 

 though Wheeler ('89) failed to find the pole-cells in the very 

 early stages of Leptinotarsa, he figures several of them (his fig. 82) 

 in a sagittal section of an egg carrying a segmented germ-band. 

 Here are shown three cells ''which are on the surface of the 

 embryo in the amniotic cavity. They are very large and clear 

 and the more anterior is apparently creeping in the manner of 

 an Amoeba, along the surface of the abdominal ectoderm. These 

 cells, the ultimate fate of which I have been unable to determine, 

 probably escape from the anal orifice of the gastrula before it 

 closes." This author also shows in transverse section (his fig. 

 87) a cell which, he says, is "about to wander through the blasto- 

 pore into the amniotic cavity." He suggests that this may be 

 the homologue of the Tolzellen.' That the cells thus described 

 by Wheeler are really pole-cells was proved by my investigations 

 on the same species. 



The embryological development of the following species of 

 Chrysomelidae was studied by Lecaillon ('98) ; Clytra laeviuscula, 

 Gastrophysa raphani, Chrysomela menthastri, Lina populi, L. 

 tremulae, Agelastica alni. In Clytra, the principal form examined, 

 Lecaillon found that the first nuclei to arrive at the posterior 

 pole of the egg became the centers of the primitive germ-cells; 

 these could be distinguished from neighboring cells by their 



