W. K. BROOKS ON THE GENUS SALPA. 69 



dome of ectoderm, as is shown at a' in Figs. 1, 2, 3 and 4, which are a 

 series of sections through a stolon a very little older than Fig. 7, parallel 

 to a line drawn from the letter c to the letter n. A stolon a little older 

 is shown in surface view in Plate XLI, Fig. 4, and in longitudinal 

 section in Plate XVI, Fig. 5. The dome of specialized ectoderm has here 

 become a deep bowl, and its growing edge pushes farther and farther 

 inwards, so as to form a double fold of ectoderm around the proximal 

 end of the stolon. In Salpa pinnata this fold soon results in the forma- 

 tion, around the base of the stolon, of a prepuce-like structure, shown in 

 Plate XLI, Fig. 6, so that, while the distal end of the stolon is covered by 

 only one layer of ectoderm, its proximal end has, as Plate XLV, Fig. 5 

 shows, three layers : first, the ectoderm, A, of the solitary salpa ; second, 

 the reflected layer, A", and third, the ectoderm, A', of the stolon. 



The portion of the stolon which is covered by these three layers of 

 ectoderm is often so much compressed that the structures included in it 

 are crowded together, as Plate XVI, Fig. 4, shows, so that the space 

 between the endodermal tube and the ectoderm appears at first sight to 

 be filled up with a mass of undifferentiated cells, although more careful 

 examination shows that all the structures which are shown in Plate 

 XXI, Fig. 7, are really present. The effect of this compression will be 

 understood by comparing this last figure with Fig. 4 of Plate XVI. The 

 presence of three layers of ectoderm around the proximal end of the 

 growing stolon gives great complexity to sections like those in Plate 

 XXI. In these figures, a is the unmodified ectoderm of the solitary 

 salpa, a' that of the stolon, and a" the reflected layer, and the relations 

 of the three layers are well shown in Fig. G, which will serve to interpret 

 the other figures. 



I do not know whether these three prepuce-like layers of ectoderm 

 are found in other species than Salpa pinnata. So far as I am aware 

 they have never been described. Seeliger gives (6), Taf. XII, Fig. 7, a 

 section of a young stolon of Salpa democratica at a stage in which he 

 says that the space between the endodermal tube and the ectoderm is 

 filled up by a solid mass of undifferentiated mesoderm. There is 

 certainly no such stage in Salpa pinnata, and it is possible that his 

 figure represents a compressed section like my Fig. 4 in Plate XVI. I 

 shall, however, discuss his view of the origin of the stolon further on. 



