POLYEMBBYONY. 



641 



I* *- 



lives. The cockroach shelters her eggs in a stout chitinous 



ootheca, the Mantis in a papery dome of hardened froth, 



the lace-winged Neuroptera protect 



their ova by supporting them on 



thin unclimbable stalks (Fig. 400, c), 



the Hymenoptera guard theirs in 



cells of wax or cement. The chief 



enemies to the eggs are certain 



species of ovivorous mites. The 



time of incubation varies from a 



few hours to, in the case of certain 



"walking-stick " insects (Phasmidae), 



two years. 



The egg is as a rule large and con- 

 tains much food-yolk. It is rounded, 

 oblate or more usually oval, and the 

 outer egg-shell or chorion may be 

 much sculptured. The anterior end 

 of the egg, that which will give rise 

 to the head, lies whilst the ovum is 

 still within the ovaries directed to- 

 wards the head of the mother. 



During the last few years the 

 phenomenon of polyembryony or 

 embryonic fission has been found to 

 occur in the families Chalcididae 

 and Proctotrypidae among the 

 Hymenoptera. Naturalists have 

 long been familiar with some re- 

 markable facts connected with the 

 life-histories of certain members of 

 the former family, but it is mainly 

 through the recent researches of 

 Marchal * and of Silvestri ^ that it 

 has been possible to give a correct 

 interpretation of them. Marchal 

 studied the biology of Encyrtus fuscicollis, a parasite of the 



* Arch. Zool. Exp. et Gen (4), ii, 1904, also Bull. Soc. Vaud. Sci. Nat. 

 1906, No. 153. 



t Rend. Ace. Lincei (5), xiv. ii., p. 534. 

 Z Til m T< 



FIG. 401. Diagrammatic median sec- 

 tion through the egg of Musca at 

 the stage of fertilization (taken 

 from drawings by Henking and 

 Blochmann). ch chorion ; d dor- 

 sal side of the egg ; dh vitelline 

 membrane ; do food-yolk ; g glu- 

 tinous cap over the micropyle ; k 

 peripheral protoplasm (periplasm 

 or perivitellus) ; TO micropyle ; 



male and female pronucleus before 

 fusion ; r polar bodies ; 

 side of the egg. 



ventral 



