352 INSECTA. 



condition in Peripatus and the Myriopoda as the more primitive, 

 following directly on that of the Annelida, while the condition of 

 the Insecta is, on the contrary, modified. 



If we are to homologise the efferent genital ducts of the Insecta 

 with those of Peripatus, we should have to trace them back to a 

 pair of transformed nephridia. Their origin from the mesoderm in 

 the Insecta would be in harmony with this ; but, in other respects, 

 we find no features retained in the development of the efferent 

 genital ducts in the Insecta which can be considered as supporting 

 such a view. 



Mention should be made of Heymons' observations that, in the genital 

 rudiment of Phyllodromia, the genital cells can from the first be distinguished 

 from the epithelial cells. This statement is not favourable to the view, until 

 now universally held, that the follicle-cells and egg-cells are derived from one- 

 and the same sort of cell by later differentiation. Regarding their first origin, 

 however, in Phyllodromia also, the two kinds of cells are to be traced to the 

 same source. 



Special attention should be called to the fact that, in the Diptera 

 and Aphidae, the genital rudiments can be recognised in a very 

 early stage of embryonic life. This is certainly to some degree 

 connected with the parthenogenetic and paedogenetic manner of 

 reproduction, which is common in these two groups, and which 

 (as in Moina, Vol. ii., pp. 123 and 180) leads to an early separation 

 of the sexual rudiments. 



In the Diptera, the first rudiment of the genital glands is repre- 

 sented by the "pole-cells." These cells (the "globules polaires " of 

 Robin, also described by Weismanx in Chironomus and Musca),* 

 which become separated at the posterior pole of the egg even before 

 the formation of the blastoderm, were discovered by Leuckart and 

 Metschnikoff (No. 55) in the asexually developing egg of the 

 viviparous larva of Ceeidomyia (Fig. 174, pz). A rather large, highly 

 granular cell (pz) here becomes severed from the posterior pole of 

 the egg (Fig. 174 D) even before the blastoderm forms, and soon 

 breaks up, first into two and then into four "pole-cells" (Fig. 

 174 F). When the blastoderm is completely formed, these "pole- 



* [It would be safer to discard Robin's term "globules polaires" for these 

 cells, since that term was also applied by him and is still applied to the polar 

 bodies or directive corpuscles, which latter, in all probability, have nothing 

 whatever to do with the "pole-cells" of Chironomus, etc., which are simply the 

 precociously separated germ -cells. This point, however, requires renewed 

 investigation, and search should be made for the probable occurrence of true 

 polar bodies and for the relationship of the " pole-cells " to the cleavage-nuclei. 

 —Ed.] 



