During the later periods of development, the resemblance of the fcetus to the adult be comes more marked daily, as well as its increase of volume compared with the pla centa. The fcetus, which in the last period referred to already showed some indication of spontaneous movement, commences to alter nately contract and dilate its body, like the adult animal. These movements, feeble at first, are before long, as the animal approaches its full term, executed with considerable vigour. The fcetus being attached to its mo ther, these movements cannot displace it; and their only end, without doubt, is to draw towards it the supply of water necessary for its respiration. As regards the stolon pro& ferum, its growth during all this time is so slow, that, even at the time of the birth of the animal, it is but a short, delicate filament. Nevertheless, on a close examination, there may be perceived on its surface a serrated edge of minute elevations, indicating the first vestiges of the buds that will subsequently be developed into embryos.
Generation in the " isolate" Salpians.— The " isolate " Salpians are, as we before mentioned, gemmiparous, and have their young produced on a small pedunculated organ, the stolon proliferum, which is visible within the single fcetus whilst still contained within its "aggre gate" mother. It then exists as a very slen der short filament, hut already gives indica tions of buds upon its surface. After the birth of the animal, it increases in size in pro portion to the continually increasing number of buds that spring from it. It is fixed by one of its extremities to the heart of the mother : and it is always at this extremity that the stolon produces new germs. The growth of the buds, just as the nutrition of the embryos, being entirely dependent on the blood of the mother, the stolon is constructed to admit a proportional quantity of the vital fluid. Two vessels traverse it throughout its length, one proceeding from the anterior extremity of the maternal heart, and the other from the oppo site end. Hence the blood, forced into one of these vessels by the contraction of the heart, returns by the other ; and at each time the heart commences to contract in an opposite direction, the two vessels quickly coincide in the change. M. Milne Edwards has demon strated that the proliferous stolons of the so cial and compound Ascidians are likewise traversed by two similar vessels, one of which has an ascending current of blood, and the other a descending current. In examining the stolon at a more advanced period of its growth, one may embrace at a view, owing to the successive germination of buds, the complete series of the phases passed through by each embryo, from the time of its first ap pearance in the form of a little button, to the full term of its development (fig. 788.). The phases passed through by the different organs correspond to those that the same organs present during the development of the " iso lated" foetus.*
Development of the fatal "aggregate" &ripe within the " sobtary."—Whatever may be the mode of aggregation of the associated Salpa' at the adult age, their germs are always dis posed in the same pattern along the stolon in two parallel rows, so that the germs alternate one with another. It necessarily follows that the embryos during growth must be arranged in the same manner. The em bryos are always placed in such a manner that the axes of their bodies cross the axis of the stolon at a right angle : they adhere among themselves by means of their organs of attach ment. The development of the "isolate," like that of the " aggregate," fcetus proceeds but slowly : the growth of those foetuses that spring from the first-formed germs is not ter minated until after the mother has almost at tained her full age. It is easily conceived that, as the number of buds continues to augment during all the time the mother grows, the form of the germs and the embryos, or the embryonic chain, acquires lastly a consider able length. Lodged in the external tunic of the mother, and adhering to the heart of the latter by the aid of the vessels of the stolon, this embryonic chain sometimes passes directly backward, and terminates before reaching the posterior extremity of the body, as in the iso lated generation of Salpa pinnata and some allied species : sometimes, as in most other species, it curls itself around the visceral nucleus, describing several spiral turns, and terminates at the anterior extremity of the body. If we examine the embryonic chain at this period, we may observe three very distinct groups of embryos (fig. 788.). The proximal group (f, e) is made up of the germs and of the embryos, as yet but little deve loped, that succeed them. These present a progressive series of the early phases of em bryonic development ; but the next group (d, e) is composed of embryos much further developed, and these being nearly all of the same size, offer but a slight trace of gradation. The distal group of embryos (b, a) having arrived almost at their full growth, present no great difference among themselves. The em bryos, products of the same stolons, leave the mother in groups, and the group most deve loped is necessarily the first to be born. The perfect uniformity in the size of the newly born individuals explains also why the ani mals of Salpa-chains are all of the same size and form. The embryonic chain of S. pinnata, however, and allied species, never presents these distinct groupings. Here, on the contrary, the phases of development proceed regularly, following the order of pro gression throughout the chain. Hence the newly born animals, grouped in a circle, are often somewhat unequal in size ; but this irre gularity soon disappears.