306 ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
the ascidiozooids in this foetus, their upper extremities do not rise so. 
high as the level of the middle of the ellipsoid formed by the cyatho- 
zooid and ovisac. The point at which their atrial apertures will 
eventually be formed, consequently, can hardly be so high as the 
lower end of that ellipsoid. 
As has been already hinted, with the advance of the foetuses in 
size all their relations become changed. The ascidiozooids, instead 
of presenting a fraction of the length of the combined ellipsoidal 
cyathozooid and ovisac, and occupying only a small portion of the 
mass of the fcetal spheroid, gradually become fully thrice as high 
as the ellipsoid in question, and form by far the greater proportion of 
the mass of the spheroid (fig. 15). The ovisac and cyathozooid, again, 
diminish, not only relatively but absolutely (fig. 16), inasmuch as 
their largest diameter does not eventually amount to more than jth 
or sth of an inch, while the lip and the internal cavity of the cyatho- 
zooid become less distinct structures than before. 
But the most curious change is that which has taken place in the 
test in the vicinity of the cyathozooid. It has, as it were, separated 
itself from the latter, following the ascidiozooids as their vertical 
diameter increases, whereby the central tongue of the test is pulled 
completely out of the mouth of the cyathozooid, as one might pull a 
finger out of a glove (compare figs. 18 and 19, Plate XXXI.) 
[Plate 30]. As a consequence of this operation a cavity, which 
eradually increases in dimensions, is developed between the outer 
surface of the cyathozooid and the inner wall of the test ; and as the 
atrial ends of the ascidiozooids ascend in consequence of the growth 
of the latter, they open into this cavity, which thus manifests itself 
as the cloaca (fig. 19). The tongue-like prolongation of the test 
becoming pulled out and flattened as the cloaca widens, ultimately 
ceases to project into the cavity of the latter, and becomes converted 
into the lip of its aperture. In fig. 1g it still protrudes for some 
distance into the cloacal cavity. 
2. The Ascidtozootds—From their small size, flattened form and 
general opacity, it is by no means so easy to trace satisfactorily the 
successive changes by which the other segments of the blastoderm 
are converted into perfect ascidiozooids, as it is to follow out the 
development of the buds. Nevertheless, knowing the latter process, 
it is not difficult to interpret the appearances presented by the seg- 
ments of the blastoderm, in the course of their development. 
When the blastoderm first becomes marked out into those seg- 
ments which eventually constitute the ascidiozooids (Plate XXXI. 
[Plate 30] figs. 11 and 12), each segment is about z3sth of an inch 
