72 JOHNS HOPKINS UNIVERSITY MOEPHOLOGICAL MONOGRAPHS. 



with indefinite outlines, separated from each other on the middle line by 

 a deep furrow, the floor of which consists of flat, sharply defined cells, like 

 those of the pharynx in general. 



At the stage shown in Plate XX, Fig. 7, where the ectoderm of the 

 stolon forms a shallow bowl or dome, the endoderm grows down into the 

 cavity of the bowl, as shown in the transverse sections in Figs. 1, 2 and 

 3, to form the endodermal tube, which is at first widely open above into 

 the cavity, c, of the pharynx, and consists of a floor of flattened, sharply 

 defined cells, which is part of the floor of the endostylic furrow of the 

 solitary embryo, and which crosses the middle line of the stolon to join 

 at each end a vertical wall of thickened cells, with indistinct and irregular 

 inner ends, like the cells in the endostylic ridges. As Fig. 1 shows, these 

 side walls of the endodermal tube are actually part of the endostylic 

 ridges, into which they can be traced in Fig. 1. Soon, however, as the 

 older or more distal section in Fig. 2 shows, the endostylic ridge proper 

 becomes separated from the endostylic side wall of the stolon by an inter- 

 vening area at d-d in Fig. 2, where the cells become flat and sharply 

 defined. In a still more distal or older section, Fig. 3, these two belts of 

 flattened cells are pushed towards each other until they meet on the 

 middle line to form the roof of the endodermal tube. The way in which 

 this tube is formed shows that it is actually a section of the endostyle 

 which drops down out of the solitary embryo into the stolon. 



As the solitary embryo develops, its two endostylic ridges come 

 nearer to each other, and finally meet on the middle line, as the succes- 

 sive stages, in Plate XXI, Plate XVI, Fig. 4 and Plate XLV, Fig. 5 show, 

 and the channel which connects the cavity of the endodermal tube with 

 that of the pharynx of the solitary salpa is thus reduced in size until it 

 becomes a small slit, d' of Plate XXI, Fig. 1. 



Within the stolon the endodermal tube assumes the shape shown in 

 the figures in Plate XXI. In cross-section it is a little like a letter H , 

 and is made up of a horizontal cross-bar and two vertical ends. In young 

 stolons the floor and roof meet and come into contact with each other 

 on the middle line, so that the cavity is divided into two lateral chambers 

 which communicate with each other only at the distal end, Plate XX, 

 Fig. 4, but as the stolon grows older the roof and the floor separate from 

 each other, as Plate XLV, Fig. 5 shows, until in mature stolons the 

 cavity assumes the shape which is shown in Plate XXXIV, Fig. 1, d'. 



