216 PROFESSOR HUXLEY ON THE ANATOMY 
In the present bud (fig. 21) this aperture is situated on the neural side of the body, in 
front of the posterior end, which is chiefly occupied by the genitalia; but as develop- 
ment goes on, the mid-atrium increases disproportionately, and encroaches upon the other 
organs, upwards and forwards, in such a manner that its anterior wall invests the whole 
posterior and lateral faces of the gastro-intestinal division of the alimentary canal ; while 
its roof (to speak metaphorically) thrusts the genitalia altogether into the haemal region of 
the body, and its posterior and inferior walls, extending backwards, carry the external tunic 
with them, and eventually cause the atrial aperture to take its place at that extremity of the 
body which is directly opposed to the mouth, and far behind the genitalia (see figs. 22-25). 
The communicating apertures between the mid -atrium and the lateral atria increase in 
size pari passu with the growth of the parts ; and hence, in the fully formed ascidiozooid, 
the gastro-intestinal division of the alimentary canal is enclosed in a sort of vertical mesen- 
tery (formed by the anterior wall of the mid-atrium in the middle line, and the internal 
wall of the lateral atria at the sides), whose layers are continued, on either hand, into the 
outer wall of the branchial sac. At the anterior boundary of the branchial sac they are 
reflected into the outer or parietal layer of the lateral atrium. 
The facts which I have detailed * are exceedingly important for the comprehension of 
Ascidian structure in general. Prom its mode of development, it is perfectly obvious that 
the inner wall of the branchial sac of Fyrosoma is not composed of tentacles which have 
coalesced, but that it is, originally, a simple imperforate dilatation of the pharyngeal por- 
tion of the alimentary canal. The development of the atrium adds a second or outer wall 
to this dilatation ; and when, by the formation of this double wall, the branchial sac is 
constituted, the stigmata make their appearance in its parietes — the atrial and the pha- 
ryngeal walls becoming united around the margins of each stigma. 
When a bud has attained a length of between iVth and ^-th of an inch, the narrow 
neck connecting it with the peduncle is obliterated, and it lies free in the general test 
of the parent ascidiarium. It next elongates until its oral and atrial apertures are placed 
in connexion with the exterior and the cloaca respectively (the latter connexion appear- 
to be effected first), and then it increases in depth until it acquires the appearance of 
the adult. Before it is detached, however, the portion of the peduncle nearest it 
and assumes the shape of a new bud ; so that the proximal end of the peduncle now 
passes into a small bud with whose apex a larger one is connected (fig. 22). And I 
suspect that this process is repeated as long as there is any reserve of generative blas- 
tema in the parental organism. I have, however, never actually seen more than two 
buds thus connected together. As the buds are all developed from the hajmal region of the 
pre-existing ascidiozooids, it follows that the new ascidiozooids formed by gemmation must 
at first be thrust among the old ones, towards the apical end of the ascidiarium. 
So much in elucidation of the mode in which the buds attain the form and general 
arrangement of organs characteristic of the adult. I now proceed to speak of such among 
the minor changes which these organs undergo as call for particular remark. 
* The accurate Krohn, in his account of the development of Phallusia (Miiller's " Archiv," 1852), was the first to 
note the separate origin and subsequent confluence of the lateral atria. In this genus, however, each lateral atrium 
has, at first, a distinct external aperture. 
