136 MEDICAL ENTOMOLOGY 



compress all the other abdominal contents into the anterior and dorsal 

 portion of the cavity, at the same time distending the abdominal wall 

 and stretching the membrane which unites the dorsal and ventral plates. 

 The mature ovaries of the mosquito appear as conspicuous white bodies, 

 easily seen through the ventral and lateral walls of the abdomen. In 

 newly-hatched females, on the other hand, the ovaries are very small 

 and inconspicuous. 



Each ovarian tube is divided into a number of chambers or follicles, 

 each of which produces one egg. There are usually four or five follicles 

 to be seen in each tube, and these show a progressively advanced stage of 

 development from the apical filament downwards, the uppermost one 

 consisting only of a small collection of undifferentiated cells, while the 

 lowest follicle contains a well developed ovum. When one batch of 

 eggs is laid the next lowest follicle comes to maturity and takes its 

 place, and so on in rotation during the life of the fly. 



The structure of the follicles is best explained by following the 

 development from its early stages. In the newly-hatched fly they 



_^ , are very small and delicate structures, consisting only 



Structure of follicles , , , , 



of a number of large cells at the upper end of the 



ovarian tube. Out of this mass three different parts arise. The 

 central cells enlarge, and one of them, the future ovum, passes to 

 the lower part of the mass ; the rest, which are to become the nurse 

 cells whose duty it is to nourish the ovum, pass to the upper pole. 

 The remainder become changed into a layer of regular cubical cells, 

 which form a covering for the ovum and its nurse cells. A number 

 of cells from the original mass remain at the upper pole, outside 

 the epithelial layer. The ovum and the nurse cells now begin to 

 show marked alterations. The nurse cells become greatly enlarged, 

 and have very large nuclei, while the ovum accumulates around its 

 nucleus a quantity of granular material, which, gradually increasing in 

 amount, forces the nurse cells still further towards the upper pole, 

 until the ovum comes to occupy the greater part of the space in the 

 follicle. The epithelium remains as a complete layer covering the 

 whole. The granular food material of the ovum soon obscures the 

 nucleus, so that it can no longer be made out in fresh preparations. 

 When the egg is ripe the epithelial layer forms a part of its covering, 

 while the nurse cells disappear entirely. 



While this is going on the small mass of cells left outside the 

 first follicle also commences to undergo a similar series of changes, 

 but as it starts after the first follicle it is always smaller. It in turn 



