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 forwai'ds, 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 hiemal 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 

 Avail 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 jiarietal layer of the lateral atrium. 



The facts which I have detailed * are exceedingly important for the comprehension of 

 Ascidian structure in general. Erom 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 Avails becoming united around the margins of each stigma. 



When a bud has attained a length of between -xVth and -g^-th of an inch, the narrow 

 neck connecting it witli 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- 

 ing to Idc eff'ected first), and then it increases in depth untU it acqtdres the appearance of 

 the adult. Before it is detached, however, the portion of the peduncle nearest it enlarges 

 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, hoAvever, never actually seen more than tAvo 

 buds thus connected together. As the buds are all develojDcd from the htemal region of the 

 pre-existing ascidiozooids, it foUoAvs that the new ascidiozooids formed by gemmation must 

 at fijst be thrust among the old ones, toAvards 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. 



