Embryo-genesis of the Simple examining under the microscope a portion of the ovary of a recent Ascidia, eggs in all stages of development may often be observed. In describing the various conditions and modifi cations of the ova of this family in course of development, we shall borrow largely from the careful and extensive researches given to the world by Professor Van Beneden, to whom naturalists are so deeply indebted for much valuable information on the subject of the em bryo-genesis of many of the Polypffera* and other groups of animals. The earliest form of the ovum is a simple vesicle; it next appears as a vesicle with an inner vesicle, which is evidently "the vesicle of Purkinje ;" and, thirdly, with a second, still smaller vesicle, " the vesicle of Wagner ;" these latter vesicles, one within the other, being enveloped in the outer or vitelline membrane (fig. 781. A).
The space between the external membrane and Purkinje's vesicle is occupied by a sub stance, at first clear and probably fluid, but soon appearing granulated, constituting the vitellus. This yolk increases rapidly, soon oc cupying the whole ovum : at the periphery its granules or cells become organised (fig. 781. a), uniting among themselves, and forming a continuous sacciform membrane. The mode of growth of the vitellus is by new cells being formed in the interior of the large older cells, and subsequently producing others in their own interior. The growth of the whole ovum is evidently carried on by this process.
The whole surface of the vitellus soon after wards becomes embossed, presenting a mul berry-like appearance (fig. 781. c). The ger minal vesicles disappear. At the centre of each little mammilla of the surface a trans parent vesicle becomes distinctly visible, and the whole appear like so many ova each with the vesicle of Purkinje. These mammillm in crease, and their peripheral substance rapidly becomes granulated, in a similar manner to the change that took place in the whole vi tellus. The mammillated appearance is now soon lost from the growth of the granules, and the surface becomes uniform, the mam millm of the periphery uniting into a mem brane, which constitutes the blastoderm. Ac cording to Milne-Edwards, the blastoderm in the Botryllidse is formed at a determinate point only ; but in Cynthia ampulla (the species in which Van Beneden watched the modifica tions of the ovum) the latter observer feels assured that it is formed simultaneously all over the yolk, as in all the inferior animals.
At this period a number of vesicles become somewhat regularly arranged over the exterior of the ovum, which by their union with one another form a new enveloping membrane. Beneath this membrane a space, occupied by fluid, soon becomes apparent. All over the exterior of this new membrane are distributed white, transparent vesicles, having much the appearance of oil globules. This envelope soon becomes more and more extended by the increase of the contained albuminous fluid, and a second membrane appears beneath it, on the surface of which also oil-like globules become apparent, as on the external mem brane (fig. 781. to). The ovum is now com posed of an external membrane, of a white fluid layer, another thin transparent membrane, an organised layer forming the periphery of the vitellus, and lastly of the soft yolk in the centre, the vitelline membrane and the blas toderm forming one with the vitelline mass.
The external membrane and the albuminous layer beneath it appear to be formed exter nally to the ovum by the oviduct or the ovary; and as they envelop the ovum after its com plete development, they may be considered only as accessory parts.
It being uncertain whether the ova in which these changes take place, have been subjected to fecundation whilst still in the ovary or ovi duct, there arises the interesting question as to what extent of modification, even to the disappearance of Purkinje's vesicle, and the mulberry-like condition of the vitellus, can take place before the ovum has received the stimulus of the seminal element.
During the changes of the ovum in this first period of its development, the colour of the egg is occasionally subject to certain modifica tions. In Ascidia grossularia, the ova become of a bright red colour, and in some species no change of colour takes place. In Amarouciunz, a compound Ascidian, the ova change from a pale to a deep yellow.
The second period of development is marked by the prolongation of one side of the yolk to form the caudal appendage. The ovum has now a chorion, albumen, and yolk, the two latter being separated by a fine membrane. The vitelline or embryonic mass becomes con tracted about its middle, and is somewhat bean-shaped. From this time there are ob servable two extremities, one lengthening it self, as the other becomes more and more globular; and respectively representing the caudal appendage and the body (fig. 781. E).