98 INSECT PHYSIOLOGY 



nourished, at first by special 'nurse cells', and then by the transfer of 

 materials from the blood by the follicular cells, until they become 

 laden with yolk. 



Oogenesis and oviposition 



In the primitive 'panoistic' type of ovary, as in Periplaneta, in which 

 nurse cells are wanting, ribonucleic acid is synthesized by the oocyte 

 itself through the action of the oocyte nucleus. All the nutritive 

 materials are supplied by the follicular cells. In the more advanced 

 'meroistic' types of ovary, as in Lepidoptera and Diptera, where each 

 oocyte has a group of nurse cells enclosed with it in its follicle ('poly- 

 trophic' group), or in Hemiptera, where the nurse cells are confined 

 to the apex of the ovary and are connected to the developing oocytes 

 by long nutritive cords ('acrotrophic' group), the nurse cells are 

 mainly responsible for the supply of ribonucleic acid which can be 

 seen streaming into the oocytes in the early stages of their develop- 

 ment. Proteins are mostly preformed in the fat body, set free into the 

 haemolymph, and transmitted to the egg by the follicular cells ; these 

 cells doubtless play a part also in the synthesis of the lipid and carbo- 

 hydrate components of the yolk. 



The follicular cells then lay down the chorion or egg shell. Finally 

 the follicle ruptures, 'ovulation' occurs and the fully formed egg is 

 discharged into the oviduct and so to the vagina. Arising from the 

 vagina there is usually a duct leading to a sac or 'spermatheca' in 

 which the spermatozoa received from the male are stored. There is 

 sometimes a dilatation or invagination of the vagina (the 'bursa 

 copulatrix') ; and there are nearly always 'accessory glands'. 



Besides the chorion laid down by the follicular cells, the egg is 

 enclosed by inner membranes which are deposited by the oocyte it- 

 self. These may amount to no more than a thin layer of wax which 

 serves to waterproof the egg, and a more or less delicate 'vitelline 

 membrane' beneath; or the blastoderm of the developing embryo 

 may lay down a tough chitinous layer, similar to the endocuticle of 

 the free living insect, which often forms the most substantial cover- 

 ing of the egg. These layers may be provided with an elaborate res- 

 piratory system which conducts oxygen to the surface of the yolk. 

 In addition, the chorion is generally pierced by one or more fine 



