638 



CLASS IV. INSECTA. 



muscular vagina which opens to the exterior. At the anterior 

 end of each ovarian tubule the cells are undifferentiated. As 

 they approach the exterior they fall into groups (Fig. 397). In 

 the simplest case as in the Cockroach (i) where there are no 

 nutritive cells, the eggs lie in a row one behind another and 

 each egg is surrounded by a layer of minute cells, which secrete 

 the chorion or thick outer cover of the egg which is pierced by 



FIG. 397. a egg tube of Forficula. Nz nutritive cells ; Ez ovum ; OE epithelium of the 

 wall of the egg tube, b median part of the egg tube of a moth. Nz nutritive cells of 

 the yolk-chamber ; Ez ovum in the germ-chamber ; H connective tissue investment, 

 so-called serosa. c egg-tube of Aphis platanoides with three ovarian chambers (Ez-Ez") 

 and the terminal nutritive chamber with its cells Nz ; Ds yolk cord. 



a micropyle for the entrance of the spermatozoon. A more usual 

 arrangement is (ii) where, between two consecutive egg-cells, 

 there lies a mass of nutritive cells which are gradually absorbed 

 as the egg approaches the vagina. This is found for example in 

 Dytiscus. A third condition (iii) is that in Rhizotrogus where the 

 ends of the ducts are swollen and form a nutritive chamber in 

 which the eggs absorb food as they pass through it. 



Certain accessory glands usually open into the vagina and 

 secrete such structures as the " cocoon " which surrounds the 

 batches of eggs of the cockroach. A spermatheca is also often 

 found and has as a rule an opening distinct from that of the 

 vagina. In the case of bees the spermatozoa retain their 



