W. K. BROOKS ON THE GENUS SALPA. 93 



left side of the stolon, Fig. 9, 28, is the rudiment of its left half. The 

 right perithoracic vesicle, Fig. 8, g, is the rudiment of the right half of 

 the cloaca ; and the one on the left, Fig. 7, h, the rudiment of its left 

 half. Plate XV, Fig. 11, is part of a horizontal section on the left side of 

 the stolon, showing these changes in a still more advanced stage. The 

 ectodermal folds have now penetrated to a considerable depth, and they 

 appear from the sections to be the active agents in the formation of the 

 endodermal pockets as well as the cloacal vesicles. At the stage shown 

 in Plate XXXIV the perithoracic tubes lie outside the endodermal 

 thickenings, and the cloacal vesicles have for a time the same positions, 

 as is shown in Plate XXIII, Figs. 7 and 8, g and h ; but as the endodermal 

 pockets, Plate XV, Fig. 11, 28, grow deeper, their blind ends push past 

 the cloacal vesicles, h, on their distal sides, so that the cloacal vesicles 

 themselves come to lie nearer the base of the stolon than the endodermal 

 pockets. Fig. 11 shows this for the left side of the stolon, and the 

 arrangement of the parts is exactly the same on the right side. In this 

 figure, as in all the others, the bottom is proximal or towards the base of 

 the stolon, and the top distal or towards its tip. The changes which we 

 have described are accompanied by important changes in other parts of 

 the stolon, which must now be noticed. They are shown in Plate XV, 

 and also in Plate V, Fig. 1. Like all the subsequent changes, they are 

 of such a character that they cannot be understood or described with- 

 out sections in at least two planes. I have therefore figured sections at 

 each successive stage from two stolons, of which one was cut parallel to 

 the long axis of the stolon, and transverse to the bodies of the salpse. A 

 series of these sections is shown in Plate XV, Figs. 1 to 10. Fig. 1 is 

 close to the germinal surface of the stolon; Fig. 2, a little higher up, 

 through the chain of eggs; Figs. 3, 4 and 5, through the lower blood 

 tube ; Figs. 6, 7 and 8, through the endodermal tube ; Fig. 9, through the 

 upper blood-tube, and Fig. 10, through the neural surface of the stolon. 

 In all these figures the top is distal and the bottom proximal, and the 

 right side of the stolon on the right. 



Another stolon was cut into sections transverse to the long axis of 

 the stolon, or parallel to the long axes of the salpa3, but instead of making 

 separate drawings of these sections, I have superimposed the drawings 

 and have thus constructed solid figures. Plate V, Fig. 1, is one of these 

 reconstructions, to show the stage which is shown in Plate XV. It is 

 a dorsal or proximal view and shows the surface which is below in the 

 sections in Plate XV. 



