214 PROFESSOR HUXLEY ON THE ANATOMY 



up by the young ovisac. Kg. 27 represents an advance upon this condition, — tlie two 

 principal changes to he noted being, first, the conversion of the lateral masses of indif- 

 ferent tissue into hollow oval bodies containing a very small cavity, and, secondly, the 

 prolongation of the posterior cornua of the alimentary sac. 



It is obvioiis that if either of these buds were viewed sideways, the middle fold of the 

 anterior face of the sac would appear like an anterior thickening, while the posterior pro- 

 longed cornua would simulate a posterior enlargement, and the whole would closely re- 

 semble fig. 19 ; and it now becomes important to prove, by the study of more advanced 

 stages, the nature of the sac, of its anterior median fold, of its posterior cornua, and after- 

 wards of the lateral sacs. Fig. 28 represents a larger bud, which presents more of its 

 anterior, than of its upper, aspect to the eye. The anterior fold, consequently, is repre- 

 sented by only two dark streaks with a clear interval. The posterior cornua are more 

 elongated than before. There is a rounded opake spot at the anterior end of the bud. 

 In fig. 29, the dark streaks have become the middle bands of the endostyle; the anterior 

 spot is obviously the nascent oral aperture ; whUe the posterior cornua have become sepa- 

 rated from the rest of the cavity of the sac, communicating with it by only a small aper- 

 ture. The cornua are, at present, of equal size ; but in fig. 30 the left cornu exhibits a 

 trihedral dilatation, which is obviously the commencement of the stomach, while the right 

 cornu and the part which joins it with the left have become, the latter the foundation of 

 the arch of the intestine, and the former of the rectum. There can be no doubt, therefore, 

 that the greater part of the sac into which the prolongation of the endostyle is developed 

 becomes what would in other animals be called the pharynx, namely, that portion of the 

 alimentary canal which lies between the oesophagus and the mouth*. 



Side views, such as those given in figs. 20 & 21, of buds, in similar stages of develop- 

 ment, are equally instructive. The attached apex of the sac is seen to become the posterior 

 ■end, or cone, of the endostyle. The small size of the gastro-intestinal, in proportion to 

 the pharyngeal portion of the alimentary tract, and the free communication of the latter 

 with the prolongation of the endostylic cone of the parent, which traverses the peduncle, 

 are clearly seen ; and I entertain no doubt that, by means of the last-mentioned communi- 

 cation, the cavity of the pharynx (or, as we shaU see it becomes, that of the branchial sac of 

 the bud) is placed in communication with that of the parent. In favourable lateral views 

 of buds in this stage, it is easily made out that the wall of the pharynx unites with the 

 external tunic at its anterior end, and here gives rise to the oral aperture, whose tentacular 

 fringe is only subsequently developed. 



Having traced the fate of the sac-like dilatation of the prolonged portion of the endostyle 

 thus far, I will now direct the attention of the reader to the coincident progress of the lateral 

 sacs, or oval hollow bodies, as I previously termed them. In fig. 27 they are very small 

 and thick -walled ; in fig. 28 then' cavity is larger, their walls are proportionally thinner, 



* The development of the Ascidian pharynx, as traced out by Krohu m Phallusia, and by myself in the present 

 memoir, appears to me to aiford ample demonstration of the justice of Milne-Edwards' s view, that the branchial sac of 

 the Ascidian is the homologue of the pharynx of the Polyzoon. It is due to my friend Prof. Allman, however, to 

 refer to what he has said in support of another theory, in his able essay " On the Homology of the Organs of the 

 Tvnicata and the Pobjzoa" Transactions of the Royal Irish Academy, vol. xxii. 



