INSECTA. 231 



ment of the embryo (if, indeed, the parent can be said to be concerned 

 in that supply which is the result rather of a series of spontaneous 

 fissions with an inherent power of assimilation of the primitive germ 

 itself), but, in some cases, the parent, having selected a fit place for 

 the deposition of her precious burthen, continues the maternal office 

 by placing near the ovum the kind of food which the larva will neces- 

 sarily require in order to complete its growth. 



Some insects, as bees and ants, feed the larva; supply them with 

 the required food from time to time, as nurses satisfy the cravings of a 

 child; bat these cares rarely devolve upon the mother in the insect class: 

 they are performed by a distinct race of individuals, of the feminine 

 sex, but incapable themselves of exercising the procreative faculty. 



The forms of the eggs of insects are very variable ; often beautiful 

 and regular, like the seeds of plants ; sometimes very singular ; always 

 perfectly adapted to the required conditions for the development of 

 the future insect. The eggs are cylindrical in Bomhyx everia ; 

 conical, with tuberculate ribs, in Pontia napi ; hemispherical in 

 Bomhyx dumeti ; lenticular in Noctua psi ; cup-shaped in Orgyia 

 antiqua ; flask-shaped in Culex pipiens ; petiolate in Hemerobius 

 perla; provided with diverging processes like ears in Scatophaga 

 putrisy to prevent their sinking too deep in the soft dung ; provided 

 with a special adaptation for floating in some aquatic insects ; with 

 numerous other modifications. 



When embryonic development begins, the vitellus becomes con- 

 densed, as in the Ascaris, receding a little from the vitelline membrane 

 at its poles. The usual processes of subdivision take place, but in 

 so much greater a degree at the peripheral layer that the subdivided 

 vitelline mass becomes invested by a stratum of minute and nucleated 

 cells. Kolliker, who has observed these early stages of insect deve- 

 lopment in the Chironomus tricinctus Schrank, gives the following 

 account of the process. The primordial cells, at first round, and 

 provided with one nucleolus, become afterwards elliptical, and ge- 

 nerally two nucleoli can be discerned in them ; afterwards two cells 

 exist, of smaller size than the parent cell. He concludes that this 

 fissiparous generation of cells, which accords with that observed by 

 Siebold and Bagge in the Ascaris, is the general mode of their mul- 

 tiplication. " Haec omnia, etsi nunquam cellulas in aliis inclusas 

 off'endi, ne ad sententiam adducunt, posteriores a prioribus gigni, ita 

 semper binae in unaquaque cellula matre oriantur." * 



The vitelline mass becomes elongated and vermiform, and, by 

 farther subdivision and coalescence of the peripheric stratum of cells 



* Observationes de prima Insectorum Genesi. 4to. Turici. 1 842. 

 Q 4 



