Section Eleven Unequally Large Rounded Bodies Measuring From

0.004 - 0.009 mm. may be distinguished. They lay as if in a multilocular gap in the germ mass, which however they did not quite fill. In each of these bodies there appeared another but far smaller body. These aggregations were distinguished from the germ by an especially beautiful intense violet gold chloride colouration of their elements. The smaller elements contained in the larger were still more intensely coloured than the larger.

Footnote 108: Loc. cit. pp. 410, 411, &c.

He further states that these aggregations equal the segments in number, and that the small bodies within the elements are not always to be seen with the same distinctness.

Oellacher's description as well as his figures of these bodies leaves no doubt in my mind that they are exactly similar bodies to those which I have already spoken of as nuclei, and the characteristic features of which I have shortly mentioned, and shall describe more fully at a later stage. A moderately full description of them is to be found in my preliminary paper.

Footnote 109: Loc. cit. p. 415. [This Edition, p. 64.]

Their division into a series of separate areas each with a deeply-stained body, as well as the staining of the whole of them, exactly corresponds to what I have found. That each is a single nucleus is quite certain, though their knobbed form might occasionally lead to the view of their being divided. This knobbed condition, observed by Oellacher as well as myself, certainly supports the view, that they are in the act of budding off fresh nuclei. Oellacher conceives, that the areas into which these nuclei are divided represent a series of separate bodies--this according to my observations is not the case. Nuclei of the same form have already been described in Nephelis, and are probably not very rare. They pass by insensible gradations into ordinary nuclei with numerous granules.

One marked feature of the segmentation of the Elasmobranch egg is the continuous advance of the process of segmentation into the yolk and the assimilation of this into the germ by the direct formation of fresh segments out of it. Into the significance of this feature I intend to enter fully hereafter; but it is interesting to notice that Oellacher's descriptions point to a similar feature in the segmentation of Osseous Fish. This however consists chiefly in the formation of fresh segments from the lower parts of the germinal disc which in Osseous Fish is more distinctly marked off from the food-yolk than in Elasmobranchii.

I conclude my description of the segmentation by a short account of what other investigators have written about its features in these fishes. One of the earliest descriptions of this process was given by Leydig. To his description of the germinal disc, I have already done full justice.

Footnote 110: Rochen u. Haie. It is here mentioned that Coste observed the segmentation in these fishes.

In the first stage of segmentation which he observed 20-30 segments were already visible on the surface. In each of these he recognized a nucleus but no nucleolus.

He rightly states that the segments have no membrane, and describes the yolk-spherules which fill them.

The next investigator is Gerbe. I have unfortunately been unable to refer to this elaborate paper, but I gather from an abstract that M. Gerbe has given a careful description of the external features of segmentation.

Footnote 111: "Recherches sur la segmentation des products adventifs de l'oeuf des Plagiostomes et particulièrement des Raies." Robin, Journal de l'Anatomie et de la Physiologie, p. 609, 1872.

Schenk has also made important investigations on the subject. He considers that the ovum is invested with a very delicate membrane. This membrane I have failed to find a trace of, and agree with Leydig in denying its existence. Schenk further found that after impregnation, but before segmentation, the germinal disc divided itself into two layers, an upper and a lower. Between the two a cavity made its appearance which Schenk looks upon as the segmentation cavity. Segmentation commences in the upper of the two layers, but Schenk does not give a precise account of the fate of the lower. I have had no opportunity of investigating the impregnated ovum before the commencement of segmentation, but my observations upon the early stages of this process render it clear that no division of the germinal disc exists subsequently to the commencement of segmentation, and that the cavity discovered by Schenk can have no connection whatever with the segmentation cavity. I am indeed inclined to look upon this cavity as an artificial product. I have myself met with somewhat similar appearances, after the completion of segmentation, which were caused by the non-penetration of my hardening reagent beyond a certain point.

Footnote 112: "Die Eier von Raja quadrimaculata innerhalb der Eileiter." Sitz. der k. Akad. Wien. Vol. LXXIII. 1873.

Footnote 113: Loc. cit. My denial of the existence of this membrane naturally applies only to the egg after impregnation, and to the genera Scyllium and Pristiurus.

Without attempting absolutely to explain the appearances described by Professor Schenk, I think that his observations ought to be repeated, either by himself or some other competent observer.

Several further facts are recorded by Professor Schenk in his interesting paper. He states that immediately after impregnation, the germinal disc presents towards the yolk a strongly convex surface, and that at a later period, but still before the commencement of segmentation, this becomes flattened out. He has further detected amoeboid movements in the disc at the same period. As to the changes of the germinal disc during segmentation, his paper contains no facts of importance.

Next in point of time to the paper of Schenk, is my own preliminary account of the development of the Elasmobranch Fishes. In this a large number of the facts here described in full are briefly alluded to.

Footnote 114: Loc. cit.

The last author who has investigated the segmentation in Elasmobranchii, is Dr Alexander Schultz. He merely states that he has observed the segmentation, and confirms Professor Schenk's statements about the amoeboid movements of the germinal disc.

Footnote 115: "Die Embryonal Anlage der Selachier. Vorläufige Mittheilung," Centralblatt f. Med. Wiss. No. 33, 1875.

EXPLANATION OF PLATE 6.

Fig. 1. Section through the germinal disc of a ripe ovarian ovum of the Skate. gv. germinal vesicle.

Fig. 2. Surface-view of a germinal disc with two furrows.

Figs. 3, 4, 5. Surface-views of three germinal discs in different stages of segmentation.

Fig. 6. Section through the germinal disc represented in fig 3. n. nucleus; x. edge of germinal disc. The engraver has not accurately copied my original drawings in respect to the structure of the segmentation furrows.

Figs. 6a and 6b. Two furrows of the same germinal disc more highly magnified.

Fig. 6c. A nucleus from the same germinal disc highly magnified.

Fig. 7. Section through a germinal disc of the same age as that represented in fig. 4. n. nucleus; nx. modified nucleus; nx´. modified nucleus of the yolk; f. furrow appearing in the yolk around the germinal disc.

Figs. 7a, 7b, 7c. Three segments with modified nuclei from the same germinal disc.

Fig. 8. Section through a somewhat older germinal disc. ep. epiblast; n´. nuclei of yolk.

Figs. 8a, 8b, 8c. Modified nuclei from the yolk from the same germinal disc.

Fig. 8d. Segment in the act of division from the same germinal disc.

Fig. 9. Section through a germinal disc in which the segmentation is completed. It shews the larger collection of cells at the embryonic end of the germinal disc than at the non-embryonic. ep. epiblast.