414 MORGAN. [Vol. V. 



isolated cells on the outer wall of each side of the stomach quite 

 near to each other, and subsequently I suppose they unite to 

 form the body cavity. Fig. 30 is from one of the youngest 

 stages found, and shows only two cells {be 2 ) outside of the wall. 

 The cells which have thus collected on the outer wall soon 

 arrange themselves around a central point or line. This seems 

 to be accomplished by the peripheral cells pushing inwards 

 under the more central cells, i.e. between the central cells and 

 the walls of the digestive tract, and ultimately arranging them- 

 selves somewhat as shown in Fig. 31 {be 2 ). At this time, all 

 essential connection with the wall of the digestive tract is lost, 

 and the increase in size of the body cavities is entirely at the 

 expense of these few granular cells which have thus early been 

 separated from the walls of the digestive tract. The later stages 

 of this pair of body cavities will be spoken of farther on. 



The origin of the third paired cavities is simpler and more 

 clearly made out than the preceding. The earliest stage I have 

 seen is shown in Fig. 32 {be 2 ). There arises on each side at the 

 level of the diaphragm-like partition separating the stomach 

 from the intestines, and underneath the preceding body cavity, 

 a solid evagination of the intestine. Each is from the begin- 

 ning composed of two walls. Very early these evaginations 

 pinch off from the intestine, and form, as in the second 

 pair of body cavities, a solid mass of cells arranged around a 

 central point. Also, as in the preceding case, the third pair of 

 permanent body cavities is formed entirely at the expense of 

 these few cells. In origin the third body cavities are undoubt- 

 edly to be regarded as gut pouches or enteroccels, but it must 

 be remembered that at first these are solid, and only second- 

 arily acquire a lumen. This may have some connection with 

 the fact that the animal is feeding during the time when the 

 pouches are formed, and thus food matter is prevented from 

 entering the pouch as it might were it hollow. The second pair 

 of body cavities does not so obviously arise as a pouch from the 

 endoderm, but may be considered an abbreviated condition, and 

 might be referred back to the same general process. 



Returning to the surface views, we have in Fig. 6, PI. XXIV., 

 a figure of a Tornaria at its most fully developed stage ; that is, 

 at a time just prior to its transformation into Balanoglossus. 

 The figure is turned in an opposite direction from Fig. 5, but 



