THE OVUM. 39 



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 ovi- 

 cluctal extremity of the egg-tube is approached 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 ti'avels 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. 



Secondly, the fact that the epithelium grows in between the separate 

 ova appears to render it almost certain that this part of the epithelium 

 must travel down the egg-tubes with the ova. 



Thirdly, the epithelium no doubt gives rise to the chorion, and considering 

 the pecidiar structure of the chorion, this seems possible only on the view 

 that the epithelium travels down the egg-tube with the ova. 



Fourthly, when, or even before, the egg is laid the epithelium under- 

 goes atrophy, and the remains of it have been compared to the corpora 

 lutea. 



If the view about the epithelium here adopted is correct, the epithelium 

 without doubt corresponds to the follicular epithelium of other ova, and has 

 the same origin as the ova themselves. 



The ovaries with yolk-cells differ in appearance from those 

 without, mainly in each ovarian chamber of an egg-tube con- 

 taining two elements, usually more or less distinctly separated. 

 These two elements are (1) at the lower end of the chamber, the 

 ovum, and (2) at the upper, large cells which gradually disappear as 

 the ovum grows larger (fig. 17 B). 



The uppermost part of the egg-tube is formed, as in the previous 

 type, by a mass of nucleated protoplasm, but the germinal cells 

 formed from it do not all become ova. The germinal cells leave the 

 germogen in batches, and in each batch one of the cells may usually 

 be distinguished from the very first as the ovum ; the remainder 

 forming the nutritive cells. In the uppermost part of the egg-tube 

 the whole mass of each batch is very small, and the successive 

 batches are very imperfectly constricted from each other. Gradually 

 however both the nutritive cells and the ovum grow in size, and then 

 as a rule, the Diptera forming a marked exception, the chamber 

 containing a batch becomes constricted into an upper section with 

 the nutritive cells and a lower one with the ovum. The ovum in 



