The development of the yolk appears to me to present special difficulties, owing to the fact pointed out by His[391] that the conditions of development vary greatly according to whether the ovary is in a state of repose or of active development. I do not feel satisfied with my results on this subject, but believe there is still much to be made out. Observations on the yolk spherules may be made either in living ova, in ova hardened in osmic acid, or in ova hardened in picric or chromic acids. The two latter reagents, as well as alcohol, are however unfavourable for the purpose of this study, since by their action the yolk spherules appear frequently to be broken up and otherwise altered. This has to some extent occurred in Pl. 25, fig. 21, and the peculiar appearance of the yolk of this ovum is in part due to the action of the reagent. On the whole I have found osmic acid the most suitable reagent for the study of the yolk, since without breaking up the developing spherules, it stains them of a deep black colour. The yolk spherules commence to be formed in ova, of not more than 0.06 mm. in the ovaries of moderately old females. In young females they are apparently not formed in such small ova. They arise as extremely minute, highly refracting particles, in a stratum of protoplasm some little way below the surface, and are always most numerous at the pole opposite the germinal vesicle. Their general arrangement is very much that figured and described by Allen Thomson in Gasterosteus[392], and by Gegenbaur and Eimer in young Reptilian ova. In section they naturally appear as a ring, their general mode of distribution being fairly typically represented on Pl. 25, fig. 27. The ovum represented in fig. 27 was 0.5 mm. in diameter, and the yolk spherules were already largely developed; in smaller ova they are far less numerous, though arranged in a similar fashion. The developing yolk spherules are not uniformly distributed but are collected in peculiar little masses or aggregations (Pl. 25, fig. 21). These resemble the granular masses, figured by His (loc. cit. Pl. 4, fig. 33) in the Salmon, and may be compared with the aggregations figured by Götte in his monograph on Bombinator igneus (Pl. 1, fig. 9). It deserves to be especially noted, that when the yolk spherules are first formed, the peripheral layer of the ovum is entirely free from them, a feature which is however apt to be lost in ova hardened in picric acid (Pl. 25, fig. 21). Two points about the spherules appear clearly to point to their being developed in the protoplasm of the ovum, and not in the follicular epithelium. (1) That they do not make their appearance in the superficial stratum of the ovum. (2) That no yolk spherules are present in the cells of the follicular epithelium, in which they could not fail to be detected, owing to the deep colour they assume on being treated with osmic acid.

It need scarcely be said that the yolk spherules at this stage are not cells, and have indeed no resemblance to cells. They would probably be regarded by His as spherules of fatty material, unrelated to the true food yolk.

As the ova become larger the granules of the peripheral layer before mentioned gradually assume the character of the yolk spheres of the adult, and at the same time spread towards the centre of the egg. Not having worked at fresh specimens, I cannot give a full account of the growth of the spherules; but am of opinion that Gegenbaur's account is probably correct, according to which the spheres at first present gradually grow and develop into vesicles, in the interior of which solid bodies (nuclei of His?) appear and form the permanent yolk spheres. When the yolk spheres are still very small they have the typical oblong form[393] of the ripe ovum, and this form is acquired while the centre of the ovum is still free from them.

The growth of the yolk appears mainly due to the increase in size and number of the individual yolk spheres. Even when the ovum is quite filled with large yolk spheres, the granular protoplastic network of the earlier stages is still present, and serves to hold together the constituents of the yolk. In the cortical layer of nearly ripe ova, the yolk has a somewhat different character to that which it exhibits in the deeper layers, chiefly owing to the presence of certain delicate granular (in hardened specimens) bodies, whose nature I do not understand, and to special yolk spheres rather larger than the ordinary, provided with numerous smaller spherules in their interior, which are probably destined in the course of time to become free and to form ordinary yolk spheres.

The mode of formation of the yolk spheres above described appears to me to be the normal, and possibly the only one. Certain peculiar structures have, however, come under my notice, which may perhaps be connected with the formation of the yolk. One of these resembles the bodies described by Eimer[394] as “Dotterschorfe.” I have only met these bodies in a single instance in ova of 0.6 mm., from the ovary (in active growth) of a specimen of Scy. canicula 23 inches in length. In this instance they consisted of homogeneous clear bodies (not bounded by any membrane) of somewhat irregular shape, though usually more or less oval, and rarely more than 0.02 mm. in their longest diameter. They were very numerous in the peripheral layer of the ovum, but quite absent in the centre, and also not found outside the ovum (as they appear to be in Reptilia). Yolk granules formed in the normal way, and staining deeply by osmic acid, were present, but the “Dotterschorfe” presented a marked contrast to the remainder of the ovum, in being absolutely unstained by osmic acid, and indeed they appeared more like a modified form of vacuole than any definite body. Their general appearance in Scyllium may be gathered from Eimer's figure 8, Pl. 11, though they were much more numerous than represented in that figure, and confined to the periphery of the ovum.

Dr Eimer describes a much earlier condition of these structures, in which they form a clear shell enclosing a central dark nucleus. This stage I have not met with, nor can I see any grounds for connecting these bodies with the formation of the yolk, and the fact of their not staining with osmic acid is strongly opposed to this view of their function. Dr Eimer does not appear to me to bring forward any satisfactory proof that they are in any way related to the formation of the yolk, but wishes to connect them with the peculiar body, well known as the yolk nucleus, which is found in the Amphibian ovum[395].

Another peculiar body found in the ova may be mentioned here, though it more probably belongs to the germinal vesicle than to the yolk. It has only been met with in the vitellus of some of the medium sized ova of a young female. Examples of this body are represented on Pl. 25, fig. 25A, x. As a rule there is only one in each of the ova in which they are present, but there may be as many as four. They consist of small vesicles with a very thick doubly contoured membrane, which are filled with numerous deeply staining spherical granules. At times they contain a vacuole. Some of the larger of them are not very much smaller than the germinal vesicle of their ovum, while the smallest of them present a striking resemblance to the nucleoli (fig. 25B), which makes me think that they may possibly be nucleoli which have made their way out of the germinal vesicle. I have not found them in the late stages or large ova.

The following measurements shew the size of some of these bodies in relation to the germinal vesicle and ovum:—

Diameter of
Ovum.		Diameter of
Germinal Vesicle.		Diameter of
Body in Vitellus.
0.096 mm.0.03 mm.0.009 mm.
0.064 mm.0.025 mm.0.012 mm.
⎧0.019 mm.
0.096 mm.0.03 mm.⎨
⎩0.003 mm.

Germinal vesicle.—Gegenbaur[396] finds the germinal vesicle completely homogeneous and without the trace of a germinal spot. In Raja granules or vesicles may appear as artificial products, and in Acanthias even in the fresh condition isolated vesicles or masses of such may be present. To these structures he attributes no importance.