In the course of incubation, the ovum in creases in size, becoming flatter and more oval. The vitelline mass becomes more com pact, and its surface denser; and the latter seems to be organised into a membrane, dis tinct from the yolk beneath. The two por tions of the ovum, before described, become more and more separate; that which occupies the centre of the ovum becomes ovoid, and knotty at one of its extremities; near the other extremity, which is continuous with the mar ginal portion, are seen one or two blackish minute points. This ring-like portion is now seen to be a caudal prolongation, too short completely to encircle the central part, from which also it is distinctly divided a little in front. Lastly, the whitish substance sur rounding the vitellus, and constituting the tegumentary mass, increases considerably in thickness.
When the ova more nearly approach ma turity, the tail-part of the vitellus is shortened, and its central part, or body of the embryo, is more and more condensed. Its anterior ex tremity becomes lobulated and encircled with a series of five cylindrical processes, which have a divergent arrangement, and advance towards the border of the egg. Three of these ap pendages terminate in a kind of button, and the intervening two are tapering anteriorly.
At each side of the base of the group of ten tacles, a little prominent lobular process is present. Lastly, the side of the body, opposite to that on which the tail is placed, becomes somewhat strongly embossed near its pos terior extremity, and towards the space where the above-mentioned black points occur (fig. 784. a).
The ovum ready for exclusion differs ap parently little from the foregoing state. The two anterior styliform appendages have almost entirely disappeared, and the three obtuse processes are further developed. The trunk is contracted towards its anterior extremity, and the yolk-mass still further condensed at the centre of the ovum.
The external membrane of the ovum becomes excessively thin, and then breaks and allows the embryo to escape. Generally this ex clusion takes place in the interior of the cloaca, but sometimes not until the ova have passed out by the anal orifice. However that may be, the young animal, free from its envelopes, soon extends its tail, and swims in the ambient fluid by the aid of its undulatory movements. In its general form the young Ascidian re sembles somewhat that of a newly born tad pole (fig. 784. c); but it still more resembles a
Cercaria. The trunk or body of the larval Poly clinum is oval and rather depressed. The whitish tissue of the future integument occupies all the surface, and is considerably developed at the margins ; its substance is granular and apparently subgelatinous ; its consistence is greatest at the surface; and it does not appear to possess a membranous investment. To wards the centre of the trunk is a large ellip tical membranous sac, the internal tunic of Milne-Edwards ; this is filled with the yellow substance of the vitellus, and is continuous anteriorly with the three tubes dilated at the end, and terminating at the anterior wall of the egg in a sort of sucker. By means of gentle graduated pressure, some of the yolk may be easily made to pass from the principal sac into these appendages, and vice versa ; the little capsule, also, terminating each of these appendages can be made to protrude exter nally by the same means. At the base of these three processes the vestiges of the others formerly occupying the intervening spaces may be observed. The yellow substance contained in the internal tunic appears to be separated into two portions; the one is clear, and situated near the wall of this sac, and the other, denser and of a deeper tint, occupies the centre. Pos teriorly a little marginal space, clearer than the neighbouring parts, is also visible, and on one of the sides the above-mentioned minute black spots are visible. The tail is very large, and, like the trunk, is composed of two distinct parts ; the one superficial, colourless, diapha nous, gelatinous, and much resembling the al bumen of the eggs of frogs ; the other, central and yellow. This latter part is continuous anteriorly with the central sac of the trunk, and is also composed of a membranous tunic, enclosing a yellow granular and semi-fluid substance. It sometimes appeared to have a central canal. The larva', after swimming about with an active wriggling motion for a few hours, attach themselves to the surface of a solid body, and, if disturbed from their position, swim about as before until they meet with a similar situation. Their activity having ceased, they become permanently fixed, and are then about the size of the head of a very small pin. They appear to affix themselves to their rest ing place by means of one of the little suckers with which their anterior extremity is fur nished.