This peculiar extra-vascular circulation, so well described by is also very distinctly seen in the Clavellinw. The learned professor especially notices C. nana as highly illustrative of this phenomenon, the inter abdominal space being in this case very large, and the currents of the nutritive fluid, with its suspended spherical globules, being easily i discernible through the transparent ntegu ments.
Mr. Bowerbank has favoured me with an account of some observations made by him on the circulation in the common tegurzientary mass of Botryllus. Under the microscope, the more transparent portion of the test ex hibited a reticulated arrangement of sangui ferous channels or vessels ; perhaps the true " marginal vessels " of Savigny ; each mesh, formed by the anastomosing currents, being occupied by one of the star-like" systems" of animals. No communication could be traced between the circulation and that of the ani mals themselves ; the former appearing to be analogous to the peculiar stem-circulation of the polypifera, to which also the circulation of the nutrient fluid in the budless stems of Perophora, noticed by Mr. Lister, vide supra', has reference.
Embryogenesis of the Botryllithn.—Tn the de velopment of the ova of the compound Asci dians, there are certain striking differences from the conditions that take place during the em bryo-genesis of the Ascidiaden. These, however, are chiefly confined to the composition of the egg, the formation of the Blastoderm, the mode of growth of the caudal appendage, the organs of vision, and the anterior appendages. From the elaborate observations of this family, given by Professor Milne-Edwards in his Paper before referred to*, it appears that the ova of several species of the Polyclinina are, whilst still enclosed in the ovary, and before that their development is much ad vanced, of an ellipsoid form, and are com posed of a very thin external membrane, a subgelatinous whitish and granular inner mass, and a minute central vesicle filled with a watery fluid. The internal vesicle is the vesicle of Purkinje, or the proligerous vesicle ; the granular substance surrounding the ve sicle is the imperfect vitellus, the vitelline membrane being the external envelope.
Whilst these ova are still enclosed in the upper part of the post-abdomen, they grow rapidly and become spherical. But the most remarkable change that takes place consists in the colour of the vitellus, which is at first a pale, and afterwards a deep yellow. The vesicle of Purkinje is still visible at the com mencement of this period of the development, but it soon disappears, and there then appears on the surface of the vitellus a nebulose spot of pale yellow, which appears to be the blasto derm or proligerous layer destined to become the embryo of the young Ascidian.
The ova arrive in the cloaca, and sometimes are even lodged in the lateral portions of the thoracic chamber, without having undergone any other appreciable modification. M. Milne Edwards considers it probable that the fecun dation of the ova takes place in the interior of this cavity. They are here brought into contact with the spermatozoa, and very shortly after having arrived here, they exhibit evi dences of active internal changes.
The granules composing the vitelline mass become grouped into clusters, forming them selves, as it were, into balls, and giving the surface an embossed or mulberry-like aspect. At the same time there is formed between the yolk and the external membrane of the ovum, a gelatinous, transparent, and nearly colourless layer, which apparently becomes the external tunic or test of the young animal.
When the ova, lodged in the marsupial pouch or cloaca, have arrived at a rather more advanced period of their development, the vi tellus loses the mulberry appearance it had but lately put on, and, if compressed between two slips of glass, it is seen to be wholly com posed of minute globules or granules of dif ferent sizes. The ovum is soon a little flat tened, and the yolk appears to be concentrated towards the middle, forming an ovoid mass of a deep yellow colour, surrounded by a some what large border of a clearer tint. This mar ginal portion of the yolk is condensed in its turn, and although at first sight appearing to constitute a sort of ring, becomes a long taper ing prolongation, which encircles the central part of the yolk, adhering by its base, and having its pointed extremity free (fig. 784. A).