1236 



TUNICATA. 



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 foetal " isolate " Salpcs. 

 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 foetus 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 foetus. 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 foetus, 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 foetus acquires a better determined form, 

 that the two orifices of the body become 

 visible, the posterior at an earlier period than 

 the anterior. The foetus 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 bands, 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 foetus 

 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 foetus 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. 



During the later periods of development, 

 the resemblance of the foetus to the adult be- 

 comes more marked daily, as well as its 

 increase of volume compared with the pla- 

 centa. The foetus, 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 foetus 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 proli- 

 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 

 foetus whilst still contained within its " aggre- 

 gate " mother. It then exists as a very slen- 

 der short filament, but 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 



