STRUCTURE AND DEVELOPMENT OF VERTEBRATE OVARY, 427 



to show that the tubuliferous tissue is in any way concerned in 

 supplying the latter form of cell. 



In a stage; seven days after birth, the same layers in the germinal 

 epithelium may be noticed as in the last described stage. The out- 

 ermost layer or pseudo-epithelium contains numerous developing 

 ova, for the most part with modified nuclei. It is separated by a 

 well marked layer of connective tissue from the middle layer 

 of the germinal epithelium. The outer part of the middle layer 

 contains more connective tissue and smaller nests than in the 

 earlier stage, and most of the cells of this layer contain modified 

 nuclei. In a few nests the protoplasm of the developing ova 

 forms a continuous mass, not divided into distinct cells, but in the 

 majority of instances the outline of each ovum can be distinctly 

 traced. In addition to the cells destined to become ova, there 

 are present in these nests other cells, which will clearly form the 

 follicular epithelium. A typical nest from the middle layer is 

 represented on Plate XIX, fig. 39 a. 



The nests or masses of ova in the innermost layer are for the 

 most part still very large, but, in addition to the nests, a few isolated 

 ova, enclosed in follicles, are to be seen. 



A fairly typical nest, selected to show the formation of the follicle, 

 is represented on Plate XIX, fig. 39 b. 



The nest contains (1) fully formed permanent ova, completely 

 or wholly enclosed in a follicle. (3) Smaller ova, not enclosed 

 in a follicle. (3) Smallish cells with modified nuclei of doubtful 

 destination. (4) Small cells obviously about to form follicular 

 epithelium. 



The inspection of a single such nest is to my mind a satisfac- 

 tory proof that the follicular epithelium takes its origin from the 

 germinal epithelium and not from the stroma or tubuliferous 

 tissue. The several categories of elements observable in such a 

 nest deserve a careful description. 



(1) The large ova in their follicles.— 'YheBt ova have precisely 

 the character of the young ova in Elasmobranchs. They are 

 provided with a granular body invested by a delicate, though dis- 

 tinct membrane. Their nucleus is large and clear, but traversed 

 by the network so fully described for Elasmobranchs. The cells 

 of their follicular epithelium have obviously the same character as 

 many other small cells of the nest. Two points about them 

 deserve notice — {a) that many of them are fairly columnar. This is 

 characteristic only of the first formed follicles. In the later formed 

 follicles the cells are always flat and spindle-shaped in section. In 

 this difi'erence between the early and late formed foUicles Mammals 

 agree with Elasmobranchs. (h) The cells of the follicle are much 

 more columnar towards the inner side than towards the outer. Thia 

 point also is common to Mammals and Elasmobranchs. 



