94 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



When the ectodermal ridges and furrows first appear on the stolon 

 they are restricted to the regions of the perithoracic tubes, but as the 

 folds grow deeper they also extend up and down, and soon completely 

 encircle the stolon, dividing its surface up into a series of complete rings, 

 each of which marks out the body of a salpa. At the stage shown in 

 Plate XV the rings encircle the stolon, as the figures show, although the 

 ectodermal folds penetrate much deeper into the stolon in some places 

 than they do in others. In Plate V, Fig. 1, the shaded area around the 

 periphery shows the extent of the infolded ectoderm, and comparison of 

 this figure with the sections in Plate XV will show its relations much 

 better than words. On the neural surface, Fig. 10, it has grown inwards 

 so far that it has cut up the nerve-tube into a series of ganglia, s, and 

 has pushed down around the ganglion in such a way as to shut it into a 

 pocket of ectoderm, open towards the upper haemal tube. The active 

 agent in the transformation of the nerve tube into a series of hollow 

 vesicular ganglia seems to be the growing ectodermal fold. Below the 

 ganglion there is a region, cut by section 9, where the ectodermal folds 

 are very superficial and faintly marked ; while still lower down in the 

 region of the endodermal tube, d', they are very deep, as shown in Figs. 

 8, 7, 6 and 5 ; still lower down, in the region of the lower blood-tube, i, 

 they are slight and superficial, Figs. 4 and 3 ; while upon the germinal 

 surface of the stolon, Fig. 1, they are very deep, so that they divide the 

 genital string into a series of partially separated segments, which are 

 about half shut in to ectodermal pockets. In general, the ectodermal 

 folds are, as Plate V, Fig. 1 shows, deepest in those regions where the 

 stolon contains solid structures, and most superficial where it is hollow, 

 although each fold forms a complete ring. The structures shown in 

 Plate V, Fig. 1, are the rudiments of a single salpa seen in proximal 

 view, with a circular groove-like body cavity, which opens on all sides 

 into the upper and lower blood spaces, / and j, and communicates 

 through them with the body cavities of the salpae before and behind it 

 in the series, as is shown in Plate XV, Fig. 4. This body cavity con- 

 tains a closed vesicular ganglion, Fig. 10, s ; two closed cloacal vesicles, 

 a right one, Figs. 7 and 8, g, and a left one, Figs. 5 and 6, h ; and two 

 vertical endodermal pockets, which form the rudiments of the right 

 half, Fig. 8, 27, and the left half, Figs. 7 and 6, 28, of the branchial sac. 

 These pockets open into the endodermal tube, Plate V, Fig. 1, d', of 

 the stolon, and through this communicate with each other, and also 

 with the branchial sacs of the salpae before and behind them in the 



