THE OVUM. 49 



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 peculiar 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 (i) 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 pre- 

 vious 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 con- 

 stricted into an upper section with the nutritive cells and a lower 

 one with the ovum. The ovum in passing down the tube be- 

 comes gradually invested by a layer of epithelial cells, which in 

 many cases pass in and partially separate the ovum from the 

 nutritive cells. The epithelium appears not unfrequently to be 

 continued as a flat layer between the nutritive cells and the wall 

 of the egg-tube. 



As was first shewn by Huxley and Lubbock, the protoplasm of the 

 ovum is often continued up as a solid cord, which terminates freely between 

 the nutritive cells, and serves to bring to the ovum the material elaborated 

 by them. It is present in its most primitive form in the somewhat 



B. II. 4 



