48 . INSECTA. 



Hal layer which passes in and forms more or less complete 

 septa between the successive chambers. The points which 

 have been especially controverted are (i) the relation of the 

 ovum to the germogen, and (2) the relation of the nutritive 

 or yolk-cells to the ovum. To the controversies on these points 

 it will only be possible to give a passing allusion. 



As has been already hinted there are two distinct types 

 of ovaries, viz. those without the so-called nutritive or yolk- 

 cells and those with them 1 . 



The formation of the ovum is most simple in the type 

 without yolk-cells, which will for that reason be first considered 

 (fig. 17 A). 



The germogen is constituted of a number of nuclei imbedded 

 in a scanty cementing protoplasm. In the lower part of the 

 germogen the nuclei are larger, and become separated off from 

 the nucleated protoplasm above, as distinct cells with a thin 

 layer of protoplasm round the germinal vesicle. These cells 

 are the ova. As they pass down the egg-tube their protoplasm 

 increases in bulk, and they become isolated by ingrowths of the 

 epithelial cells the origin of which is still uncertain, which form 

 round each ovum a special follicle, so that the egg-tube is filled 

 by a single row of ova each in an epithelial follicle (fig. 17 A). 

 The larger the ova the more columnar is the epithelium of the 

 follicle. As the oviductal extremity of the egg-tube is ap- 

 proached the ova increase in size, and their protoplasm is more 

 and more filled with yolk particles. 



In the lower part of the egg-tube the epithelium gives rise to 

 a chorion. 



The epithelium around each ovum has been spoken of as forming a 

 follicle, and it is implied that the epithelium round each ovum travels down 

 the egg-tube with the ovum. It is however by no means clear from the 

 observations of the majority of writers that this is the case, and in fact the 

 epithelium is generally spoken of as if it were simply the epithelium of the 

 egg-tube. In favour of the view here adopted the following considerations 

 may be urged. 



Firstly, there is considerable evidence that the superficial layer of 

 the germogen gives rise to the epithelial cells, simultaneously with the 

 formation of the ova from the deeper layers. 



1 For a list of the genera with and without nutritive cells, vide Brandt, pp. 47 

 and 48 



