116 GEOEGE W. FIELD. 



forwards and increase in size until they almost completely fill 

 the cavity of the preoral lobe (figs. 24 and 29, el). The 

 appearance of the enterocoels in cross-section is shown in fig. 

 34, el. 



Meantime the posterior ends of these cylindrical tubes have 

 extended backwards and also dorsally and ventrally, so that 

 they come to overlie the stomach and intestine. Ventrally 

 and posteriorly the right and left enterocoels fuse together. 

 On the left side, just posterior to the pore canal, there early 

 appears a constriction (fig. 18, z) which finally narrows and 

 separates the left enterocoel into two parts; the anterior, 

 opening to the exterior at its posterior end by the poie canal 

 and water-pore, extends forwards into the preoral lobe, where 

 its cavity communicates with the right enterocoel, and the 

 posterior part, the left posterior enterocoel, whose cavity is 

 ventrally in connection with the posterior part of the right 

 enterocoel. In the case of the right enterocoel I have often 

 noticed a constriction, but have never found it divided into an 

 anterior and a posterior portion as is the left enterocoel. In 

 studying the living animal it should be noticed that the walls of 

 the enterocoel are contractile, and that a temporary constriction 

 may occur at almost any point. Fig. 18 is a diagram made 

 from a reconstruction of a series of transverse sections of a 

 larva. The outline of the left enterocoel {el), previous to its 

 division into the anterior and posterior enterocoels, is marked 

 by the dotted line. It does not show the posterior ventral 

 fusion between the right and left enterocoels. The hydrocoel 

 has not yet formed. 



Further History of the Mesenchyme. 

 The amoeboid cells arising from the entodermic area press 

 into the segmentation cavity. This cavity is filled with a 

 transparent, jelly-like substance, and in this the mesenchyme- 

 cells, by their long, delicate, anastomosing processes, form a 

 network which serves for supporting the archenteron and the 

 enterocoels. Mesenchyme-cells apply themselves to the walls 

 of the body, of the digestive tract, and of the enterocoels, and 



