TUNICATA. 



1237 



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 

 lull term of its development (Jig. 788.). The 



Fig. 788. 



Salpa zonaria (aggregate) in itsfcetat state. Magni- 

 fied about 4 times. {After Eschricht.') 



a, b, part of the first set of the young Salpaj ; c, 

 d, the second set; e,f, the third set; g, the stem 

 with its germs ; h, //, the anterior orifices ; i, i, vis- 

 cera ;j,j, ganglia; ft, k, posterior orifices ; I, ves- 

 sels ; m, muscular bands of the branchial sacs. 



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 foetal "aggregate" Salpa; 

 within the "solitary." Whatever may be the 

 mode of aggregation of the associated Salpae 

 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 bodii-s cross the axis of the 



* The development of the foetal "aggregate" 

 Salpians is beautifully illustrated in the fourth and 

 fifth plates of Prof. Eschricht's Paper on the Salpte, 

 in the Royal Trans. Copenhag. vol. viii. 1841. 



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," fetus 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 Sulpa 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 ne-irly 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. 



The embryonic chain, as we have seen, is 

 lodged in the external tunic of the mother. 

 During the earlier periods of its growth, the 

 substance of the tunic envelops it so closely, 

 that no interval is apparent between them ; 

 but, as the mass of embryos increases in size, 

 there is formed around it, and chiefly around 

 the most fully developed group of embryos, a 

 cavity which is prolonged towards the surface 

 of the mother's body, and opens externally by 

 a large orifice. By this orifice the embryos, 

 when mature, make their exit. The posi- 

 tion of this opening always corresponds with 

 the point where the embryonic chain happens 

 to terminate, which is sometimes near the 

 anterior extremity, but sometimes even at 

 the posterior extremity of the body of the 



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