61 



granules scattered through a transparent substance. They are the yolk vesicles. 

 Attentive examination shows that the yolk vesicles are contained in the original 

 protoplasm of the ovum, which everywhere extends among them, forming a network. 

 In fact the yolk vesicles are developed by the protoplasm within itself. The protoplasm 

 takes up the nutritive liquid materials derived from the blood and alters these by its 

 living chemistry so as to form these yolk vesicles which grow as it were at all points 

 throughout the substance of the protoplasm. Tu the mature ovum these yolk vesicles 

 become homogeneous and transparent, and flow together, with the exception of a few 

 of them, which remain separate but transparent near the outer surface of the ovum. 

 In the ovum now under consideration the nucleus retains the structure described in 

 the younger stage, and is still near the centre of the ovum. When the yolk vesicles 

 fuse the nucleus passes to the exterior with the protoplasm, the whole of which forms 

 a superficial layer enclosing the central semifluid yolk as in a blalder. 



We have now to consider the ovarian structures surrounding the egg at the stao-e 

 represented in Fig. 3. It is easy to recognise outside of all the thin membrane of 

 fibrous tissue connected with and belonging to the stroma of the ovary. This may be 

 called the follicular membrane, the ovarian capsule enclosing the ovum being usually 

 known as the follicle. The follicular membrane exhibits nuclei along the course of the 

 fibres composing it, and it contains a number of capillary blood vessels, communicating 

 with larger vessels in the stroma (b.v. in Fig. 3). But within the follicular membrane 

 is a regular epithelium which was not seen in the younger stage of the ovum. This is 

 composed of a single layer of cells possessing large distinct nuclei. These cells in the 

 preparation are separated from one another at their internal ends, but this is probably 

 due to the contraction caused by reagents. Such a follicular epithelium is found round 

 the ripening ova of all vertebrates. But it is still an open question whence it is 

 derived. It has been maintained by many great authorities that the follicular cells 

 are derived from the germinal epithelium ; that when on ovum separates from the 

 epithelium and sinks into the stroma it takes with it a few other cells of the epithelium, 

 Avhicli multiply by division and form the foUicular epithelium. This conclusion 

 has been drawn mainly from the study of the ovary in embryos, not from that of 

 developing ova in adult animals. As I have shown I have failed to trace any 

 connection between the follicular epithelium and the germinal epithelium. The former 

 seems to me to be entirely wanting for a considerable period after the young ovum has 

 separated from the germinal epithelium. The cells of the follicular epithelium become 

 visible about the same time as the commencement of the vitelline membrane. If these 

 cells are not derived from the germinal epithelium they must be derived either from 

 the egg itself or from the stroma of the ovary. There is no evidence of their derivation 

 from the egg itself at least in bony fishes, and it is inconsistent with what we know of 

 fibrous tissue to suppose that the nucleated fibres can produce cubical cells. The 

 follicular cells cannot be developed independently from nutritive material derived from 

 the blood, they must be descendants of previously existing cells, for we know of no 



