232 PROFESSOR HUXLEY ON THE ANATOMY 
During the whole of these changes and until the foetus attains a diameter of ^-th of 
an inch, it remains within the mid-atrium of the parent, which, at last, it completely fills. 
With Savigny, I am unahle to understand how it escapes, unless indeed it becomes freed 
bv the destruction of its parent. Tor it seems quite impossible that the foetus should find 
ay open to it by any conceivable amount of dilatation of the atrial apertur 
N 
does one ever find a fully formed ascidiozooid without a foetus in its mid-atrium. And if, 
at the same time, it is recollected that only one ovum ever comes to maturity in an 
ascidiozooid, so that when the foetus has arrived at its full development the parent's 
pation 
>> 
it seems less improbable that the destruction of the latter should 
be involved in the maturity of its offspring. 
Such is a general description of the changes in the size, form, and position of the chief 
constituents of the fetus, in virtue of which it assumes its final characters. It now becomes 
necessary to trace the internal modifications which each of these constituents undergoes. 
1. The Cyathozooid.— In my brief preliminary sketch of the development of Pyrosoma 
(' Annals of Natural History ' for January, 1860), I have termed this part the " rudi- 
mentary cloaca ; ' ' but it would have been a more accurate account of the matter, if I had 
called it the ■ mould ' or ' forerunner ' of the cloaca. Rudiment of the cloaca, in the strict 
sense of the words, it is not ; for, as we shall see, the atrial apertures of the ascidiozooids 
never really open into it. 
When the cyathozooid is first distinguishable as a separate segment and traces of 
structure are discernible in it (Plate XXXI. fig. 11), it presents, when viewed from above, 
near that edge which is most distant from the first isthmus, a rounded depression. 
Viewed sideways, the blastoderm appears to be divided into two lamellae, the separation 
between which is most marked immediately under the depression. In a line between the 
depression and the first isthmus a clear streak is visible, the first rudiment of what I 
shall term the appendix of the cyathozooid. As the development of the foetus progresses, 
the interspace between the two layers of the blastoderm enlarges and the depression 
becomes an opening, into which, however, the thick test is continued, projecting like a 
conical tongue into the interspace or cavity just mentioned, in such a manner as to leave 
but a narrow median passage, by which I conceive that a free communication between the 
cavity of the cyathozooid and the exterior must be effected (figs. 17 & 18)- At * ne 
same time, the aperture is gradually shifted from the margin to the centre of the cyatho- 
zooid, so that, eventually, its middle corresponds to one pole of the foetus (fig. 14), and gives 
the latter the appearance of a cup, or of an egg with its top cut off. Contemporaneously 
with these changes that streak which I have mentioned takes shape as a singular 
appendage situated between the two layers into which the outer wall of the cyathozooid is 
differentiated, and a communication, which, I believe, existed from the first between the 
cyathozooid and the first ascidiozooid by means of the first isthmus, becomes patent ana 
obvious. But a description of the structure of a more advanced cyathozooid will best 
render these changes intelligible. 
Pig. 14 represents a foetus ^th of an inch in diameter. The cyathozooid and ovisac, 
taken together, have the form of an ellipsoid, truncated at that end which presents 
the aperture of the cyathozooid, and rounded at the other. The circular aperture of 
the cyathozooid (/3) is ri^h of an inch across, and is bounded by a constricted perpen- 
