278 Robert Wilhelm Hegner. 



milieu de sa jDaroi interne ;" these "penetreront au milieu des cellules 

 mesodermique au moment ou le sillon se fermera." 'No pole-cell 

 canal is figured, nor does lie mention in the text by what means this 

 penetration takes place. 



Although Friederichs (1906) did not observe the formation of 

 pole-cells in Donacia crassipes he found a group of germ-cells lying 

 just within the egg at the posterior pole; these, he thinks, are derived 

 from the blastoderm. Beneath this group is an opening in the blasto- 

 derm similar to the pole-cell canal found in Calligrapha, and one 

 cannot but suspect that pole-cells arise in Donacia as in other Chryso- 

 melidse, but were overlooked by this author. The opening in the 

 blastoderm is considered the blastopore. Friederichs says of it, "Der 

 Blastoporus der Chrysomeliden liegt am hinteren Pol und wird ver- 

 schlossen durch die Genitalanlage, welche an dieser Stelle bereits 

 ensteht, sobald das Entoderm (die primiiren Dotterzellen) und das 

 primare Ektoderm gesondert sind, Der Blastoporus wird spater 

 zum After des Insekts." 



In CalUgrapJia and Leptinotarsa, a definite pole-cell canal is 

 present, homologous to the "Polzellen canal" found by Escherich in 

 Musca. The origin of this canal has been described in a previous 

 chapter (III) and the progress of the pole-cells through it was 

 there followed in detail. It undoubtedly is the opening in the blasto- 

 derm caused by the pole-cells during their formation ; this opening 

 is kept free from cells by a plug of cytoplasm connecting the group 

 of pole-cells lying in the posterior amniotic cavity with the pseudo- 

 blastodermic nuclei just within the egg (Fig. 25). The canal closes 

 only after all the pole-cells have passed through it (Fig. 40). It 

 will be shown later that in Calligrapha the pole-cells migrate into 

 the embryo by means of amoeboid movements. 



B. Tlie Migration of the Germ-Cells witliin the Embryo. 



It is difficult to ascertain in many cases whether the germ-cells 

 described by the earlier authors changed their position within the 

 embryo by means of an active migration, or were passively moved 

 about by the shifting of the other embryonic tissues. Metschnikoff 



