The constitution of the female generative organs of the Trematodes was first clearly ascertained by von Siebold (No. [12]). He originally, though not very confidently, propounded the view that the germinal vesicles alone were formed in the ovary and that the protoplasm of the ovum was supplied by the yolk-gland. This view has long been abandoned, and von Siebold (No. [13]) himself was the first to recognize that true ova with a protoplasmic body containing a germinal vesicle and germinal spot were formed in the ovary. The Trematodes have however not ceased to play an important part in forming the current views upon the development of ova, and have quite recently served Ed. van Beneden as his type in exposing his general view upon this subject.
His view consists fundamentally in regarding the secretion of the yolk-glands, which in most cases merely invests the ovum, as homologous with the yolk-spherules which fill the protoplasm of many eggs; and he considers the part of the ovary where in most forms the ova receive their supply of yolk particles, as equivalent to the vitellarium of the Platyelminthes. He further appears to regard the primitive state as that exemplified in Trematodes, Cestodes, etc., and holds that the ovarian types characteristic of other forms are secondarily derived from this, by the coalescence of the primitively distinct vitellarium with the ovary proper.
This appears to me a case of putting the cart before the horse. To my mind the vitellarium is to be regarded, as has already been suggested by Gegenbaur, Hallez, etc. as a special differentiation of the primitively simple ovarian tube, and the instances Of Macrostomum and Prorhyncus just cited appear to me to indicate some of the steps in this differentiation. In Macrostomum the cells of the lower part of the oviduct simply supply a kind of nutriment to the ovum in the form of granular yolk particles, while in Prorhyncus the yolk cells of the lower part of the ovarian tube form a complete investment of independent cells for the ovum. If this lower part of the ovarian tube were to grow out as a special diverticulum we should have produced a normal vitellarium. But even with the above modification the theory of van Beneden appears to me not completely satisfactory. The view that the yolk-spherules are of the same nature as the yolk cells is mainly supported by the case of Prostomum caledonicum, where the vitellarium produces the yolk particles which fill the ovum. The cases Of Prorhyncus and Macrostomum give a different complexion to that of Prostomum caledonicum. From the first of these especially it appears that, even when normal yolk cells surround the ovum, yolk particles can be deposited independently in the protoplasm of the ovum.
The most probable view of the nature of the vitellarium is that of Gegenbaur, Hallez, etc., according to which it is to be regarded as a specially modified part of the ovarian tube. On this view the nature and function of the yolk cells admit of a fairly simple explanation. They are to be regarded as primary germinal cells like those in the ovaries of Hydra, Tubularia, etc., which do not become converted into ova. Like these cells they may in some instances, Macrostomum, Prostomum, etc., serve directly in the nutrition of the ovum. In other cases they retain their independence and serve for the late nutrition of the embryo. In both instances they retain the faculty, normally possessed by ova, of forming yolk particles in their protoplasm.
Echinodermata.
(15) C. K. Hoffmann. “Zur Anatomie d. Echiniden u. Spatangen.” Niederländisch. Archiv f. Zoologie, Vol. I. 1871.
(16) C. K. Hoffmann. “Zur Anatomie d. Asteriden.” Niederländisch. Archiv f. Zoologie, Vol. II. 1873.
(17) H. Ludwig. “Beiträge zur Anat. d. Crinoiden.” Zeit. f. wiss. Zool., Vol. XXVIII. 1877.
(18) Joh. Müller. “Ueber d. Canal in d. Eiern d. Holothurien.” Müller’s Archiv, 1854.
(19) C. Semper. Holothurien. Leipzig, 1868.
(20) E. Selenka. Befruchtung d. Eies v. Toxopneustes variegatus, 1878.
[Vide also Ludwig (No. [4]), etc.]
The eggs of the Echinodermata present in their development certain points of interest.
The ovaries themselves are usually surrounded by a special vascular dilatation. In the Asteroidea, the Echinoidea, and the Holothuroidea the organs have the form of sacks; specially surrounded in the two former groups, and probably the latter, by a vascular sinus formed as a dilatation of one of the generative vessels. In the Crinoids they have the form of a hollow rachis completely surrounded by a blood-vessel. ([Fig. 11], b.) The proximity of the ovaries (generative organs) to the vascular system in these forms has clearly the same physiological significance as the proximity of the ovaries (generative organs) to the radial vessels in the Cœlenterata.
In the Asteroidea, the Echinoidea and the Holothuroidea the ovaries have the form of sacks lined by an epithelium of germinal cells, and the ova are formed by the enlargement of these cells, which, when they have reached a certain size, become detached from the walls, and fall into the cavity of the ovarian sack. In Toxopneustes (Selenka) and very probably in other forms only a few of the epithelial cells undergo conversion into ova: the remainder undergo repeated division, and, as in so many other cases, are eventually employed in the nutrition of the true ova. In the nearly ripe ova of Asterias Fol has described a flattened follicular epithelium the origin of which is unknown.