No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 129 



in these cases is suggested by Grassi: -Cette disposition qui 

 est nettement metamerique preserve I'ovaire du danger d'etre 

 d6t^rior6 de quelque mani^re que ce soit. Le danger existe 

 particuli^rement quand les oeufs sont pr^s d'arriver k maturite 

 et provient de ce que, chez lapyx, la differentiation segmentaire 

 de la musculature et de la cuticule est avancee au point que les 

 metam^res ont acquis une grande independance de mouvement. 

 Cette independance est beaucoup moindre chez Campodca et 

 c'est pour cela que cet insecte n'offre pas la disposition indique 

 plus haut." If this be an adequate explanation, the resemblance 

 of the sexual organs in the Thysanura to those in the Orthoptera 

 is due to secondary causes. At all events, this question must 

 remain open till Thysanuran embryos can be studied. 



The metameric mesodermal origin of the germ-cells in 

 embryo Orthoptera is too much like the origin of the germ- 

 cells in Annelids to be considered as secondary and I fully 

 agree with Heymons Cai), and Korschelt and Heider ('92) in 

 regarding the sexual organs of such forms as the Rhynchota 

 (Aphidae, Cicadida?) and Diptera {Chironomus, Cecidomyia) as 

 derived by a process of precocious segregation from metameric 

 gonads like those of the Orthoptera. These exceptional forms 

 frequently exhibit peculiar and aberrant features (partheno- 

 genesis, paedogenesis) like the Crustacea which have a similar 

 precocious segregation of germ-cells {e.g. Moina, according to 

 Grobben, '79). 



The genital ducts of the insect embryo are not so readily 

 reduced to the Annelid type. Many authorities, it is true, 

 have regarded them as modified nephridia but apart from their 

 paired mesodermal origin and tubular structure there was very 

 little to support such a view, 1 But now the prevailing view 

 receives fresh support from the fact that the ducts in both 

 sexes arise as hollow diverticula of the coelom. Though 

 temporarily obliterated the lumen of the duct is very probably 

 a persisting remnant of the coelomic cavity. This is certainly 



1 My statement in a former paper ('89) that the genital ducts might arise from 

 tracheal involutions is erroneous. What I saw and figured (Fig. 80, PI. XIX) 

 was a section through the terminal ampullae of the deferent ducts, and not as I 

 supposed, through their orifices. 



