232 PROFESSOR HUXLEY ON THE ANATOMY 



Duriuo- the Avhole of these changes and until the foetus attains a diameter of i^jth of 

 an inch, it remains within the mid-atrium of the parent, which, at last, it completely fills. 

 With Savigny, I am unable to imderstand how it escapes, imless indeed it becomes freed 

 by the destruction of its parent. For it seems quite impossible that the foetus should find 

 a way open to it by any conceivable amount of dilatation of the atrial aperture. Nor 

 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 fojtus has arrived at its full development the parent's 

 "occupation is gone," 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 foetus, 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.—ln my brief preliminary sketch of the development of Ftjrosoma 

 ('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 ' moiild ' 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 lamellge, 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 ctjathozooid. 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 the 

 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 and 

 obvious. But a description of the structure of a more advanced cyathozooid will best 

 render these changes intelligible. 



Fig. 14 represents a foetus -^gih. 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 tu jth of an inch across, and is bounded by a constricted perpen- 



