AND DEVELOPMENT OF PYROSOMA. 239 



The first tlieory of the mode of formation of the adult ascidiarium which suggests itself 

 is obviously that which supposes that the four ascidiozooids of the foetus give rise, by 

 budding, to all those of the adult Pyrosoma, at the same time losing the two stolons, and 

 acqumng reproductive organs, so as to be undistiuguishable from their agamogenetic 

 progeny. 



But difficulties arise when we compare this theoretical conception with the structural 

 characters, and the ascertained laws of gemmation of Pyrosoma. 



In every ascidiozooid of the adult ascidiarium (and there is no reason to suppose that 

 those of the tetrazooidal foetus constitute exceptions to the rule) budding takes place, as 

 we have seen, from a single definite region of the body, situated in the posterior moiety 

 of the htemal surface ; and the buds remain, in nearly the same plane as that in which 

 they were given olT, until they have attained some distance from the parent. It has been 

 seen, in fact, that three buds, given off successively from one ascidiozooid, may be visible, 

 one below the other, in the same, not very thick, longitudinal section. But in the tetra- 

 zooid, as in the adult, the hsemal side is that turned away from the aperture of the asci- 

 diarium. If, then, the buds thrown off from the ascidiozooids of the foetus all remain 

 on the hfemal, or apical, side of their parents, we ought, on examining the adult organism, 

 to find the four primitive ascidiozooids close to the margin, with a series of two or three 

 buds, in various stages of development, attached to each. 



As a matter of fact, however, no section taken near the margin of the aperture has 

 ever presented an appearance essentially different from that represented in PL XXX. 

 fig. 4. The ascidiozooids have always been young, and, on the average, younger, the 

 nearer they were to the margin. But they have never been younger than such a bud as 

 is represented in fig. 24i, PI. XXX. ; and those of the first three or four tiers have always 

 possessed imperfectly developed sexual organs, and buds not more advanced than those 

 represented in figs. 19 and 20. 



That the ascidiozooids which lie nearest the aperture are the result of the budding of 

 other ascidiozooids is beyond all doubt. As I have traced the development of the stolon 

 from such a modified bud, it is clear that the bud is not developed, as I had once 

 imagined, from the stolon of another ascidiozooid, — these stolons being invariably trace- 

 able, without a break, into the lip of the cloaca, where they end cajcally. There appears to 

 me, then, to be no other course open but to suppose that these yoimg ascidiozooids which 

 lie nearest the aperture, are buds which were originally developed from the hsemal 

 region of ascidiozooids which lie nearer the apex, and that they have consequently 

 passed round and to the neural side of their parents. If this migration of the buds reaUy 

 occurs, it will follow, as Savigny supposed, that the four apical ascidiozooids of the adult 

 are the modified zooids of the foetus,— the buds developed from their haemal waUs not 

 remaining upon their apical side, but passing up between them on to their neural sides, 

 and there becoming themselves new centres, whence fresh buds are thrown off, which 

 gradually take their places in a still higher tier. 



I can conceive of no other mode in which the structure of the foetus, the structure 

 of the adult and the law of budding can be reconciled ; and yet. I am reluctant to admit 

 so seemingly artificial a process on anything short of direct evidence. Such evidence, 



