AKtlUlDA. 407 



The walls of the vagina consist of a layer of columnar 

 epithelium, resting externally on a basement membrane 

 and lined internally by a layer of chitin (Plate III., 

 fig. 31). The median diverticulum from the vagina has 

 a closely similar structure, except that its nuclei are larger 

 and no cell boundaries are distinguishable in its epithelial 

 layer. Its lumen, on account of the flattening of the 

 organ, is extremely narrow and is in the form of a 

 transverse slit. 



A few muscle fibres are attached to the walls of the 

 vagina, and pass outwards in an oblique direction to be 

 inserted into the neighbouring body-wall. They probably 

 aid in the extrusion of the ova by bringing about the 

 alternate expansion and contraction of the cavity of the 

 vagina. 



The germinal tissue is situated in each ovary as a 

 protruding ridge lying in the region of the third and 

 fourth abdominal segments. The ridge is dorsally placed 

 in each ovary, and is inclined somewhat towards the 

 outside of the body (Plate VI., fig. 57). It consists of a 

 mass of irregularly-arranged cells, each cell with an oval 

 nucleus with the chromosomes regularly disposed around 

 its periphery. By means of rapid mitotic divisions the 

 germinal ridge buds off a mass of cells, which lie free in 

 the ovarian cavity. At an early stage two kinds of cells 

 become distinguishable, viz., vifellogenous or nutritive 

 cells, which have rounded nuclei rich in chromatin 

 material, and the egg cells or ova, which are much less 

 numerous and are scantily supplied with chromatin, 

 which is arranged in eight small granules disposed around 

 the periphery of each nucleus (fig. bb). 



There is no arrangement of the ova into follicles, but 

 a number of vitellogenous cells become grouped around 

 each ovum. As development goes on the ova accumulate 



