AND DEVELOPMENT OF PYROSOMA 
239 
v U k, uu „w «v, iu tuv.ii « a uiuus. 
The first theory 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 
acquiring reproductive organs, so as to be undistinguishable from their affamoeenctic 
progeny. 
But difficulties arise when we compare this theoretical conception with the structure 1 
characters, and the ascertained laws of gemmation of Pyrosonia. 
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 t he posterior moiet y 
of the haemal surface; and the buds remain, in nearly the same plane as that in which 
they were given off, 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 haemal 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 haemal, 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 scries 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. 24, PL 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 caecally. There appears to 
me, then, to be no other course open but to suppose that these young ascidiozooids which 
lie nearest the aperture, are buds which were originally developed irom the haemal 
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 really 
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 walls 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, 
