The observations made by MM. Lowig and KZilliker on the embryogeny of some of these animals are generally in accordance with those of M. Milne-Edwards, but on some points, and especially with regard to the develop ment of the Botrylli, there exists considerable discordance of opinion. The points involved being not only of interest, but of no slight im portance, it is necessary to give in detail some of the most important observations recorded by the above-named naturalists.
In Botryllus violaceus, B. aureus, .Apli dium gibbulosnm, and Amaroucium Nordmanni, MM. Lowig and Kalliker observed, in the first stages of development, a division of the yolk similar to that taking place in the eggs of frogs, and described also by M. Milne-Ed wards, and established also in the case of the simple Ascidians by Van Beneden. This divi sion takes place as in the intestinal worms ; that is to say, the simple nuclei, contained in the globules, which, as everywhere, are only aggregations of granules, always become double before the globules are divided in two. As soon as the division has arrived at a certain degree, the spherical form of the mass of glo bules becomes elongated, and takes more and more the form of an embryo, its tail making a semicircle about its body. The tail is distinct before any other part, and this, according to these observers, is evidently formed, not by an elongation of the embryo, but by the separa tion of a portion of the globules from the sur face. In Amarouduni and Aplidium, at the same time as the exterior form of the embryo, and at a very early period, the two ocular points, remarked by Milne-Edwards, make their ap pearance; presenting as yet no trace of an envelope or of anterior processes. At a later period only is there formed around the em bryo a transparent, colourless border, which, in Anzaroucium and Aplidium, increasing more and more, especially at the thick end of the body, appears as a very strong lamina, hut remains without structure all the time the embryos are contained in the membranes of the egg. Cotemporaneously with the forma tion of the envelopes, the embryos themselves commence also to undergo a series of changes. Firstly, towards the anterior extremity, there appear the three appendages of peculiar form ; secondly, the yellowish substance in the in terior of the body separates into two laminae; the one, external, remains diaphanous ; the other, internal, becomes opaque, and divides, in the Botrylli, into eight conical corpuscles, which surround a somewhat large, round, perforated nipple ; lastly, a great number of structural modifications take place. The mature embryos of B. aureus are formed of a spherical body, 0.28"/ broad and 0.38"/ long, possessing an orifice surrounded anteriorly with three lobules, and posteriorly bearing a thin, tapering tail, 0.72"' long. These em
bryos present exteriorly the thin, transparent, structureless layer (tegumentary layer) before referred to, from which almost solely the lobes, or lanceolate appendages of the head, are formed, and which terminate at the opposite extremity in a prolongation exceeding in length those parts of the tail enclosed within it. In terior to this, in the anterior part of the body, is a second delicate envelope, formed of cells either round or changed into fibres, which does not enter into the composition of the lobes of the head, but encases the mammil lated prominence before described, and also the eight spherical bodies surrounding it, and is attached at one end to the edge of the nipple, and at the other to the interior part of the tail.
The internal substance, constituting the chief mass of the embryos, is, according to MM. Lowig and Kolliker, evidently a group of individuals, as M. Sars, who discovered those curious embryos of the Botrylli, has already shown. The eight spherical corpuscles, united at their bases, and provided with a kind of common stem, are so many individuals, and the prominent nipple situated in their centre represents the common excretory tube. This salient tube at its extremity has three lobules, which project into the base of the lobes of the exterior envelope : from its base three filaments (nerves ?) arise, which passing upwards bifurcate each into two, one of which terminates at the orifice of the tube ; the second reaches the summits of the lo bules, and, passing beyond them, spread into five or six branches, extending almost to the edge of the lobes of the exterior envelope. In the eight embryos no orifice is perceptible, nor any other organs, except some canals (intestine) indistinctly rolled up. Their mi croscopic elements, however, are very dis tinct ; namely, various-sized nucleated cells, filled with pale red granules and fibres in process of formation ; the former constitutes the principal mass of the excretory tube. The internal part of the tail, which is apparently a direct continuation of the substance of the embryo, possesses an interior cavity, and its walls are composed of two layers of cells. The internal layer is formed of cells of 0.012"' diameter, rectangular, with the angles slightly rounded, distinctly nucleated, and containing fine yellowish granules. They are very regularly arranged side by side in transverse series, so that the cavity of the tail is always surrounded by 10-12 cells. The external layer is composed of a continuous, simple layer, formed of minute cells, measur ing 0.003'"-0.004'", without any distinct ar rangement. It is to be observed that this tail, formed simply of cells and a homogeneous envelope, exhibits very active movements, affording a new proof of the contractility of parts composed merely of simple cells.