My observations in Raja are not so full as those upon Scyllium, but they serve to complete and reconcile the observations of Semper and Schultz, and also to shew that the general mode of growth of the follicular epithelium is fundamentally the same in my representatives of the two divisions of the Elasmobranchii. In very young eggs, in conformity with the results of all previous observers, I find the follicular epithelium approximately uniform. The cells are flat, but extended so as to appear of an unexpected size in views of the surface of the follicle. This condition does not, however, last very long. A certain number of the cells enlarge considerably, others remaining smaller and flat. The differences between the larger and the smaller cells are more conspicuous in sections than in surface views, and though the distribution of the cells is somewhat irregular, it may still be predicted as an almost invariable rule that the smaller cells of the follicle will line that part of the surface of the ovum, near to which the germinal vesicle is situated. On Pl. 25, fig. 30, is shewn in section a fairly average arrangement of the follicle cells. Semper considers the larger cells of such a follicle to be probably primitive ova destined to become permanent ova. This view I cannot accept: firstly, because these cells only agree with primitive ova in being exceptionally large—the character of their nucleus, with its large nucleolus, being not very like that of a primitive ovum. Secondly, because they shade into ordinary cells of the follicle; and thirdly, because no evidence of their becoming ova has come before me, but rather the reverse, in that it seems probable that they have a definite function connected with the nutrition of the egg. To this point I shall return.
In the next stage the small cells have become still smaller. They are columnar, and are wedged in between the larger ones. No great regularity in distribution is as yet attained (Pl. 25, fig. 31). Such a regularity appears in a later stage (Pl. 25, fig. 32), which clearly corresponds with fig. 8 on Pl. 34 of Schultz's paper, and also with the stage of Scyllium in Pl. 25, fig. 29, though the distinction between the two kinds of cells is here far better marked than in Scyllium. The big cells have now become flask-shaped like those in Scyllium, and send a process down to the vitelline membrane. The smaller cells are arranged in two or three tiers, but the larger cells in a single layer. The distribution of the larger and smaller cells is in some instances very regular, as shewn in the surface view on Pl. 25, fig. 33. There can, it appears to me, be no doubt that Schultz's view of the smaller cells being lymph-cells which have migrated into the follicle cannot be maintained.
The thickness of the epithelium at this stage is about 0.04 mm. In the succeeding stages, during which the egg is rapidly growing to the colossal size which it eventually attains, the follicular epithelium does not to any great extent alter in constitution. It grows thicker on the whole, and as the vitelline membrane gradually atrophies, its lower surface becomes irregular, exhibiting somewhat flattened prominences, which project into the yolk. At the greatest height of the prominences the epithelium may reach a thickness of 0.06 mm., or even more. The arrangement of the tissues external to the follicular epithelium is the same in Raja as in Scyllium.
The most interesting point connected with the follicle, both in Scyllium and Raja and presumably in other Elasmobranchii is that its epithelium at the time when the egg is rapidly approaching maturity is composed with more or less of distinctness of two forms of cells. One of these is large flask-shaped and rich in protoplasm, the other is small, consisting of a mere film of protoplasm round a nucleus. Considering that the larger cells appear at the time of rapid growth, it is natural to interpret their presence as connected with the nutrition of the ovum. This view is supported by the observations of Eimer and Braun, on the development of Reptilian ova. In many Reptilian ova it appears from Eimer's[387] observations, that the follicular epithelium becomes several layers thick, and that a differentiation of the cells, similar to that in Elasmobranchii, takes place. The flask-shaped cells eventually undergo peculiar changes, becoming converted into a kind of beaker-cell, with prolongations through the egg membranes, which take the place of canals leading to the interior of the egg. Braun also expresses himself strongly in favour of the flask-shaped cells functioning in the nutrition of the egg[388]. That these cells in the Reptilian ova really correspond with those in Elasmobranchii appears to me clear from Eimer's figures, but I have not myself studied any Reptilian ovum. My reasons for dissenting from both Semper's and Schultz's views on the nature of the two forms of follicular cells have already been stated.
The Vitellus and the development of the yolk spherules.—Leydig, Gegenbaur, and Schultz, have recorded important observations on this head. Leydig[389] chiefly describes the peculiar characters of the yolk spherules.
Gegenbaur[390] finds in the youngest eggs fine granules, which subsequently develop into vesicles, in the interior of which the solid oval spheres, so characteristic of Elasmobranchii, are developed.
Schultz describes in the youngest ova of Torpedo the minute yolk spherules arranged in a semilunar form around the eccentric germinal vesicle. In older ova they spread through the whole. He also gives a description of their arrangement in the ripe ovum. Dr Schultz further finds in the body of the ovum peculiar protoplastic striæ, arranged as a series of pyramids, with the bases directed outwards. In the periphery of the ovum a protoplastic network is also present, which is continuous with the above-mentioned pyramidal structures.
My observations do not very greatly extend those of Gegenbaur and Schultz with reference to the development of the yolk, and closely agree with what Gegenbaur has given in the paper above quoted more fully for Aves and Reptilia than for Elasmobranchii.
In very young ova the body of the ovum is simply granular, but when it has reached about 0.5 mm. the granules are seen to be arranged in a kind of network, or sponge-work (Pl. 25, fig. 21), already spoken of in my monograph on Elasmobranch Fishes.
This network becomes more distinct in succeeding stages, especially in chromic acid specimens (Pl. 25, fig. 22), probably in part owing to a granular precipitation of the protoplasm. In the late stages, when the yolk spherules are fully developed, it is difficult to observe this network, but, as has been shewn in my monograph above quoted, it is still present after the commencement of embryonic development. An arrangement of the protoplasmic striæ like that described by Schultz has not come under my notice.