26 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



vtl< 



■egg c. 



eggc. 



The ooplasm (formative yolk) and the deutoplasm (food yolk) consti- 

 tiitc the egg yolk; the former chiefly albuminoid, the latter composed of 

 fat, lecithin, cholesterin, glj^cogen, and inor- 

 ganic salts, substances derived from the nurse 

 cells of meroistic ovaries or from the follicular 

 epithelium of panoistic ovaries. 



A meroistic ovary is one in which the egg 

 tubes (ovarioles) contain nurse cells that may 

 all be restricted to the germarium or apical part 

 of the tube (found in the Hemiptera, poly- 

 phagous Coleoptera, and Siphonaptera) or that 

 alternate with the eggs in the vitellarium (in 

 the Dermaptera, Diptera, Lepidoptera, 

 Hymenoptera, and adelphagous Coleoptera) 

 (Fig. 27). A panoistic ovary is one that lacks 

 nurse cells, as in Machilis and Japyx among 

 Apterygota and in the Odonata, Orthoptera, and 

 Plecoptera. The nurse cells are abortive eggs 

 and together with the functional eggs are de- 

 rived from the embryonic germ cells. In Mi- 

 astor, however, according to Hegner (1912) the 

 nurse cells are derived from the somatic cells. 



The part played in the formation of yolk by 

 certain inclusions such as mitochondria and 

 Golgi bodies is still a matter of controversJ^ 

 King (1926) maintains that in Peripatopsis 

 capensis yolk formation begins before the 

 mitochondria have become distributed. The 

 yolk is fatty, but its source has not been deter- 

 mined. In Drosophila melanogaster and D. 

 simulans, according to Ephrussi (1925), there 

 is no proof that the mitochondria play any part 

 in the elaboration of yolk. 



Payne (1932) describes and figures young 

 oocytes of a stone fly in which the yolk and 

 fat are at first located around the nucleus and 

 migrate peripherally, but with insects in general 

 they arise in the periphery. Nicholson (1921) 

 found that to some extent yolk material is laid 

 down in the periphery of Anopheles eggs in the 

 earliest stages, but undoubtedly the greater part 

 of the nutritive material reaches the egg through 

 the medium of the nurse cells. Although Gresson (1929) asserts that the 



nur 



fol 



Iffl 



Fig. 27. — Ovariole of 

 meroistic ovary. Alternat- 

 ing nurse cells (nur). (egg 

 c) Egg cells, (fol) Follicle. 

 iger) Germarium. {vtl) 

 Vitellarium. 



