ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 345 
than of its upper, aspect to the eye. The anterior fold, consequently, 
is represented by only two dark streaks with a clear interval. The 
posterior cornua are more elongated than before. There is a rounded 
opaque 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; while the posterior 
cornua have become separated from the rest of the cavity of the sac, 
communicating with it by only a small aperture. 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 cesophagus and the mouth ! 
Side views, such as those given in figs. 20 & 21, of buds, in similar 
stages of development, 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 communication, the cavity 
of the pharynx (or, as we shall 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 their cavity is larger, their walls are 
proportionally thinner, and the sacs themselves are both absolutely 
1 The development of the Ascidian pharynx, as traced out by Krohn in Phadlusia, and by 
myself in the present memoir, appears to me to afford 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 
Tunicata and the Polysoa,” Transactions of the Royal Irish Academy, vol. xxii. 
