380 THE ANATOMY OF INVERTEBRATED ANIMALS. 



puscles, one process of the corpuscle passing into a ramifica- 

 tion of the trachea. Schulze is inclined to think that the 

 other processes end in parenchyma cells. 



The nerves of the photogenic plates are derived from the 

 last abdominal ganglion ; they branch out between the paren- 

 chyma cells into finer and finer branches, which eventually 

 escape observation. 



The female reproductiv^e organs of insects consist of the 

 ovarian tubes, or ovarioles, with their so-called jDeritoneal in- 

 vestments, and of the oviducts, which unite into a vagina ; 

 while a spermatheca, and, generally, accessory glands open 

 into, or close to, the vagina. 



The ovarioles may be few or very numerous. Each con- 

 sists of an external structureless membrcma propria^ within 

 which lies a solid columnar mass composed of cells. The an- 

 terior, usually tapering, end of this ovarian mass is composed 

 of protoplasmic substance in which nuclei are imbedded, but 

 in which the contours of the cells which they indicate are not 

 distinguishable. Further back, some of these nuclei enlarge, 

 become surrounded by an accumulation of protoplasm, and 

 constitute the primitive ova. Each primitive ovum is sepa- 

 rated from its fellow by a layer of nucleated protoplasm 

 which thus forms a capsule around it. In some insects, such 

 as Blatta, the capsule is hardly distinguishable in those ova 

 which lie between the smallest and those of middling size, 

 which follow the former in order from before backward. But, 

 in the larger ova which succeed these, the cells of the ovicap- 

 sule rapidly enlarge in a direction perpendicular to the sur- 

 face of the ovum, and constitute a very ^veil-marked epithelial 

 layer. I am inclined to believe that, for some time, an addi- 

 tion is made to the vitellus of the egg^ by these epithelial 

 cells, and that they, in fact, play the part of vitelligenous 

 cells. But however this may be, before long, a delicate struct- 

 ureless lamella appears on the surface of the vitellus and 

 incloses the eg^g as a vitelline membrane. The epithelial 

 cells of the ovicapsule next secrete from their surface a thicker, 

 often ornamented, layer of chitinous substance, which consti- 

 tutes the chorion, and the egg is complete. 



The ovarian mass, therefore, as Waldeyer has justly pointed 

 out, corresponds w^ith one of the epithelial tubes of the ovary 

 of a vertebrated animal, and the ovicapsules answer to Graa- 

 fian follicles. 



In some insects, as Aphis, the indiff*erent tissue of the an- 

 terior end of the ovarioles gives rise not only to ova and ovi- 



