The placenta, or attachment of the foetal "iso late" in the " aggregate" &like. —The embryos of the Salpce, undergoing all the phases of de velopment within their mother, adhere to her by the aid of an organ, the use of which is to furnish them with the necessary nutritive ele ments. These elements being derived from the blood of the mother, the vessels of the latter enter largely into the composition of this organ. Its structure and form vary much according to the mode of propagation peculiar to each generation. The propagation in the "iso lated" Salpians is by gemination, and here the organ in question is the proliferous slolon. In the " aggregate " Salpian, on the other hand, this organ is represented by a body generally round, situated on the inferior surface of their single foetus, and fixed to the internal wall of the maternal cavity, precisely in the position occupied by the egg, and to this organ many naturalists have with reason attributed the functions of a placenta. This placenta is ordinarily situated in the external tunic of the fimus; its structure is far from being well known. In appearance it is a soft, whitish, or brownish pulp, traversed by nu merous vessels ; it is attached by a very short pellicle, which is formed by a prolongation proceeding from the inner tunic, and envelop ing this organ. The vessels distributed in the interior of the placenta communicate with four trunks, two of which communicate with the vascular system of the foetus, and the other two with that of the mother. Those of the foetus, in descending towards the placenta, traverse the peduncle ; the one conducting the blood to this organ, and the other re turning it to the foetus. The two maternal trunks have analogous functions ; they ter minate at the point where the placenta is attached to the mother. Each of these foetal and maternal trunks alternately act as arteries and veins. This alternation is due to the periodic changes that determine the contrac tions of the heart in Tunicates generally. The vessels of the foetus and of the mother are not in direct communication ; but, as in Manzmalia, they are merely contiguous. M. Krohn's microscopical observations also tend to prove that the maternal blood never com mingles with the foetal blood. The blood-cor puscles of the foetus are distinguishable from those of the mother by their less size and by the more constant regularity of their forms.
The development of the placenta com mences at any early stage of enbryonic life, and in its progress corresponds to that of the foetus. Ultimately the placenta becomes detached from the maternal tissues, and is carried away by the young born animal. It remains for a long time in connection with the young animal, but decreases rapidly in size, and ultimately disappears before the full growth of the animal is perfected.
Eleoblast of the festal Salpce. — There is another organ belonging to the foetus, men tioned by authors, especially Chamisso, Mey en, and Krohn, which is a round whitish body, lodged, like the placenta, in the external tunic of the foetus. Its use is entirely un known. It appears to be composed of a multitude of fascicles or lamellm, that, by their intercrossing with one another, cir cumscribe cellular cavities, filled with a per fectly clear oily liquid, composed chiefly of globules. The fascicles or lamellm are tra versed by numerous vessels opening into two trunks, which apparently form the at tachment between this organ and the visceral nucleus. Meyen thought it to be the vitelline sac of the foetus ; but, according to M. Krohn, this opinion is inadmissible, because the " aggregate" embryos, which we know are not produced from eggs, are provided with it. This body, M. Krohn terms the "eleoblast." During the incubation of the foetus, and after the birth of the individual, the changes of volume undergone by this organ correspond in general to those of the placenta ; but its decrease proceeds more slowly, and its re mains are observable long after the placenta has disappeared.
Development of the fatal "isolate" Salim e.
— The development of the foetal "isolate" Salpa occupies a considerable time, not being accomplished until the mother has almost attained her full growth. Of the progress of the genesic phenomena, there is as yet but little clearly known. The first changes mani fest in the egg after fecundation, are the early disappearance of the germinal vesicle and spot, the increase of the egg in size, and the loss of its ovular, and assumption of a spherical, form. Soon afterwards the egg, so transformed, is replaced by a round body, which raises the internal tunic of the mother into a slight nipple-like prominence, projecting into the internal cavity of the latter. This is the rudi mentary placenta, and is channelled by a cavity which is in direct communication with the two maternal vessels previously spoken of. These vessels are at this period very mi nute, but they quickly increase in size. By their means a current of blood is already es tablished in the interior of the rudimentary placenta. The maternal blood carried by one of the vessels of the cavity rises on one side towards its base, and then, describing an arch, descends on the other side, to return to the mother by the opposite vessel. The first rudiment of the fcetus is, it seems, developed after the appearance of the placenta. It is at first a very minute body, formed on the sum mit of the placenta and under its envelope : this covering is a continuation of the internal tunic of the mother, and becomes, at a later period, the external tunic of the 'Cetus. The organs soon appear in the rudimentary embry onic mass. The respiratory cavity, M. Krohn observes, is, probably, one of the first parts formed, the fcetus, previously solid, be coming evidently hollow ; immediately after wards the rudiments of the branchia and the nervous ganglion are perceptible ; the visceral nucleus, the eleoblast, and the heart become distinct only at a late period. When the eleoblast is developed, it is placed anterior to the visceral nucleus, and the heart then begins to contract, although feebly. It is only when the fcetus acquires a better determined form, that the two orifices of the body become visible, the posterior at an earlier period than the anterior. The kerns now surpasses the placenta in volume, although the latter has not, since its appearance, ceased to grow ; and the nervous ganglion, distinguishable from all other parts by its rapid growth, is conspicuous from its volume, compared to that of the other organs, and gives origin to numerous nerve-filaments. The eleoblast, the volume of which is considerably increased, tends to place itself below the nucleus. It is at this period also that we can clearly dis tinguish the muscular hands, although in a yet imperfect state. Each band is represented by two lateral symmetrical portions, separated one from another by a large interval along the superior surface of the body ; it is not until a later period that the two portions are united into a single band. The placenta also exhibits a marked change. The cavity that it con tained disappears, and its place is occupied by the pulpy whitish substance before spoken of At a more advanced stage, but still far from the full term of development, the fcetus presents a form that subsequently is but little altered. Its volume, compared with that of the placenta, is greater than before. The eleoblast, as yet less than the placenta, is now placed behind it, and soon equals it in size. The distribution of the vessels on the different parts of the fcetus is become more apparent ; and the two trunks enclosed in the peduncle of the placenta are distinguishable. Now also the contractions of the heart may be seen to change their direction periodically, the blood consequently circulating in the same singular manner as in the adult Salpians. But another still more remarkable phenomenon manifests itself at this period, viz. the precocious form ation of the stolon proliferum, which springs from near the heart, in the form of a little button.