W. K. BROOKS ON THE GENUS SALPA. 95 



series, as is shown in Plate XV, Fig. 7. At the lower end of the body 

 is an egg, n, which lies partly in the body cavity and partly in the 

 lower blood-tube, i. As Plate XV, Fig. 2, shows, it is not yet com- 

 pletely shut off from the other eggs in the series, although the follicle 

 is beginning to grow in between the eggs. Still lower down, in a 

 pocket of the body cavity, the rudiment of the testis is represented by 

 two folds of the follicle, Fig. I, m, m. Plate XXIII, Fig. 5, shows that the 

 active agency in the segmentation of the genital string is the growth of 

 the fold of ectoderm, a, and that this presses in to the genital string and 

 cuts it up just as it cuts up the nerve tube and the peri thoracic tubes. 

 The body cavity also contains scattered mesoderm cells, shown in Plate 

 XV, Fig. 11, and the rudiment of the pericardium, Fig. 7, e. At this 

 stage this is external to the outer surface of the right endodermal 

 pocket, 27, and it is probably formed from the cells which are shown 

 in Plate XXXIV, Fig. 3 and Fig. 11, between the right perithoracic tube 

 and the ectoderm. They are shown at a more advanced stage at e in 

 Plate XXIII, Fig. 8. I do not know whether there is a left pericardial 

 rudiment or not. In Fig. 5 of Plate XV, shown more enlarged in Fig. 

 12, there is a rudiment on the left side, exactly like the one shown on 

 the right side in Fig. 7, e. Salensky states (2, p. 44) that there is a left 

 pericardial rudiment as well as a right one in the rudimentary ascidio- 

 zooid of Pyrosoma, although the definitive pericardium and heart are 

 formed from the right one alone. In Salpa the heart and pericar- 

 dium are certainly formed from the right one, and if there is a left 

 one it disappears very early, for I found no trace of it in older sections, 

 and after I had drawn Figs. 5 and 111 discovered reasons for suspecting 

 that I may possibly have drawn an inverted and misplaced section, and 

 that the pericardial rudiment may possibly be only another figure of the 

 right one, which is shown in Fig. 7. 



SECTION 4. The Development of the Chain- Sdlpa. 



The vertical endodermal pockets, 27 and 28, at first open, along their 

 whole length, into the endodermal tube, d', of the stolon, but their upper 

 or oral ends soon begin to grow up inside the body cavity of the salpa 

 towards the ganglion, as is shown in Plate V, Fig. 2, so that sections 

 above the level of the endodermal tube show them as closed tubes, 

 Plate XV, Fig. 8, 28. In Fig. 9 the right side of the section cuts their 

 upper blind ends, while the section passes above them on the left. They 



