. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 316 BULLETIN OF THE atrial opening (g) in the same figures is formed in a similar manner, but somewhat later, by coalescence with the walls of the atrial chamber. The muscular bands wliich adhere to the inside of the outer tunic of the adult are derived from the atrial tunic, and will be described in connection with that organ. The Body Cavity. — During the early stages the whole surface of the branchial sac, except the region next the placenta, is surrounded by the body cavity, the connection of which with the outer
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 316 BULLETIN OF THE atrial opening (g) in the same figures is formed in a similar manner, but somewhat later, by coalescence with the walls of the atrial chamber. The muscular bands wliich adhere to the inside of the outer tunic of the adult are derived from the atrial tunic, and will be described in connection with that organ. The Body Cavity. — During the early stages the whole surface of the branchial sac, except the region next the placenta, is surrounded by the body cavity, the connection of which with the outer chamber we have already described. As development progresses, the outer tunic and the wall of the branchial sac unite at various points, and the atrial tunic is formed between them and has regions of attachment. Embryo more advanced than the one shown in Fig. 22, but less highly magnified : the muscu- lar girdles are partially separated : 1, sinus system of nurse ; 10. opening of placenta : 12t outer or foetal chamber of placenta •, c, branchial sac of nurse ; h, branchial cavity of nurse ; hi outer tunic; c'. wall of branchial sac; e, branchial aperture; h\ branchial cavity; i, mid- atrium; w, gill; m, endostyle ; /, muscles; v, ganglion; x, elaeoblast; d i, digestive or' gans. to both, so that we no longer have a single body cavity, but instead of it a sinus system. Branchial Sac and Digestive Organs.— The branchial sac is at first a single, nearly oval cavity (13, Fig. 20), occupying a little more than the anterior half of the embryo, while the posterior half is filled with a mass of cells (14, Fig. 20), which are to give rise to the various organs of the nucleus. The cavity of the branchial sac soon becomes lengthened backward, so as to occupy two thirds or three fourths of the body of the embryo (Figs. 22, IS; 23 and 24, h), and the nucleus is divided, unequally, into two portions, of which the smaller (Fig. 23, di), which is upon the neural side, gives rise to the w
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