STRUCTURE AND DEVELOPMENT OP VERTEBRATE OTARY. 423 



stage I have given two figures corresponding to those of the 

 earlier stage (fig. 36 and 36a). 



From these figures it is at once obvious that the germinal 

 epithelium has very much increased in bulk. It has a thickness 

 0*1 — 0'09 mm. as compared to 0'03 mm. in the earlier stage. 

 Its inner outline is somewhat irregular, and it is imperfectly 

 divided into lobes, which form the commencement of structures 

 nearly equivalent to the nests of the Elasmobranch ovary. 

 The lobes are not separated from each other by connective 

 tissue prolongations ; the epithelium being at this stage perfectly 

 free from any ingrowths of stroma. The cells constituting the 

 germinal epithelium have much the same character as in the 

 previous stage. They form an outer row of columnar cells internal 

 to which the cells are more rounded. Amongst them a few 

 large cells with granular nuclei, which are clearly primitive ova, 

 may now be seen, but by far the majority of the cells are fairly 

 uniform in size, and measure from 0*01 — 0"03 mm. in diameter, 

 and their nuclei from 0"004 — 0*006 mm. The nuclei of the 

 columnar outer cells measure about 0*008 mm. They are 

 what would ordinarily be called granular, though high powers 

 show that they have the usual nuclear network. There is no 

 special nucleolus. The rapid growth of the germinal epithelium 

 is due to the division of its cells, and great masses of these may 

 frequently be seen to be undergoing division at the same time. Of 

 the tissue of the ovary internal to the germinal epithelium, it may 

 be noticed that the tubuliferous tissue derived from the Malpig- 

 hian bodies is no longer in contact with the germinal epithelium, 

 but that a layer of vascular stroma is to a great extent interposed 

 between the two. The vascular stroma of the hilus has, more- 

 over, greatly increased in quantity. 



My next stage is that of a twenty six days embryo, but the 

 characters of the ovary at this stage so closely correspond with 

 those of the succeeding one at twenty eight days that, for the sake 

 of brevity, I pass over this stage in silence. 



Figs. 37 and 37a are representative sections of the ovary of the 

 twenty-eighth day corresponding with those of the earlier stages. 



Great changes have become apparent in the constitution of the 

 germinal epithelium. The vascular stroma of the ovary has 

 grown into the germinal epithelium precisely as in Elasmobranchs. 

 It appears to me clear that the change in the relations be- 

 tween the stroma and epithelium is not due to a mutual growth, 

 but entirely to the stroma, so that, as in the case of Elasmo- 

 branchs, the result of the ingrowth is that the germinal epithe- 

 lium is honeycombed by vascular stroma. The vascular growths 

 generally take the paths of the lines which separated the nests in 

 an earlier condition, and cause these nests to become the egg 



