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 : 12 t 

 outer or foetal chamber of placenta •, c, branchial sac of nurse ; h, branchial cavity of nurse ; h i 

 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 wall of 

 the digestive organs, while the larger or hamial portion (Fig. 23, x) 



