558 THE STRUCTURE AND DEVELOPMENT 



to indicate that they do not essentially differ from cells which 

 have the character of undoubted connective tissue cells, and 

 renders it possible that the greater part of the tunic, which has 

 apparently the structure of ordinary connective tissue, is in 

 reality derived from the original germinal epithelium, a view 

 which tallies with the fact that in some instances the cells of 

 the tunic appear as if about to assist in forming the follicular 

 epithelium of some of the developing ova. In Raja, the 

 similarity of the pseudo-epithelium to the subjacent tissue is 

 very much more marked than in Scyllium. The pseudo- 

 epithelium appears merely as the superficial layer of the ovarian 

 tunic somewhat modified by its position on the surface. It 

 is formed of columnar cells with vertically arranged fibres which 

 pass into the subjacent layers, and chiefly differ from the 

 ordinary fibres in that they still form parts of the cell-proto- 

 plasm enclosing the nucleus. In PL 25, fig. 34, an attempt is 

 made to represent the relations of the pseudo-epithelium to 

 the subjacent tissue in Raja. Ludwig's figures of the pseudo- 

 epithelium of the ovary, in the regular form of its constituent 

 cells, and its sharp separation by a basement membrane from 

 the tissue below, are quite unlike anything which I have met 

 with in my sections either of Raja or Scyllium. 



Close to the dorsal border of the ovary the epithelial cells of 

 the non-ovarian region have very conspicuous tails, extending 

 into a more or less homogeneous substance below, which con- 

 stitutes a peculiar form of tunic for this part of the ovarian 

 ridge. 



In the full-grown female the stroma of the ovarian region is 

 denser and has a more fibrous aspect than in the younger 

 animal. Below the pseudo-epithelium it is arranged in two or 

 three more or less definite layers, in which the fibres run at 

 right angles. It forms a definite ovarian tunic. The pseudo- 

 epithelium is much more distinct, and the tails of its cells, so 

 conspicuous in previous stages, can no longer be made out. 



Formation of the permanent ova and the follicular epithelium. 

 In my monograph on the development of Elasmobranch Fishes 

 an account was given of the earliest stages in the development 

 of the primitive ova, and I now take up their development from 



