MUSEUM OF COMPARATIVE ZOOLOGY. 317 



becomes the eltcoblast. The cavity of the oesophagus soon becomes 

 visible as a diverticulum from the posterior end of the cavity of the 

 branchial sac, and is in direct communication with the latter at the 

 earliest stage observed. Two diverticula are now formed upon the 

 lower or haemal surface of the oesophagus, side by side and parallel 

 with each other (Fig. 4, o") ; as these grow they gradually unite and 

 form a single large diverticulum, which is bent forward towards the 

 mouth so that it lies nearly parallel with the oesophagus, as shown in 

 Fig. 24, o". Since the cavity of the stomach is derived from that of 

 the branchial sac, it must, like the latter, be regarded as a portion of 

 the primitive digestive cavity of the gastrula. The cavity of the in- 

 testine, however, first appears as a closed chamber, parallel with and 

 on the haemal side of the stomach, and having no communication 

 with the cavity of the latter (Fig. 4, o"). 



The partition which separates the two soon disappears, and they be- 

 come continuous, although the anal end of the intestine, which is close 

 to the mouth, is still closed. After the chamber of the mid-atrium or 

 cloaca has been formed the anus unites with this, the partition dis- 

 appears, and the digestive organs now form an U-shaped tube, con- 

 necting the branchial with the atrial cavity. As nothing was learned 

 of the development of the so-called " hepatic organ " of the solitary 

 Salpa, I will now give what little was learned about it in the chain- 

 salpa, in order that this description of the mode of formation of the 

 various organs may be as complete as possible. In the young chain- 

 salpa, at the time it is discharged into the water, the posterior end of the 

 stomach and the side of the intestine are covered with a single layer 

 of large, nucleated, dark-colored cells (Figs. 33 and 34), which present 

 the characteristics of liver cells, and agree in appearance, and approxi- 

 mately in position also, with the liver cells upon the wall of the upper 

 portion of the stomach of a Polyzoon. 



At the time that the observations upon Salpa were made a species of 

 Appendicularia was very abundant. The wall of the stomach of this 

 bore, upon the inside, two large clusters of cells which were arranged 

 in a single layer, and agreed, in appearance, color, &c, with those found 

 upon the outside of the stomach of Salpa, which are undoubtedly liver 

 cells, and the rudimentary " tubular hepatic organ." 



The study of this organ was extremely difficult, as the eloeoblast of 

 the solitary Salpa obscures this portion of the digestive organs at all 



