W. K. BROOKS ON THE GENUS SALPA. 215 



Plate XVI, Fig. 4, is a transverse section near the middle of the 

 stolon from an older embryo, in which the bodies of the chain-salpae are 

 indicated at the extreme tip by constrictions. Plate XXXIV, Fig. 1, is a 

 transverse section through the base of a stolon from a fully grown Salpa 

 pinnata, which carried a long series of buds and had set free a great 

 number of chain-salpae. Plate XXXIV, Fig. 2, is a more enlarged trans- 

 verse section, and Fig. 4 a horizontal section of its germinal mass. 

 Plate XV, Fig. 2, is a similar horizontal section at a short distance from 

 the base of the same stolon, and other horizontal sections, farther and 

 farther from the base, are shown in Plate XXII, Figs. 1 and 2, Plate 

 XXIII, Figs. 1, 2 and 5, Plate XXVII, Figs. 1, A, B, and <7, m, Plate 

 XXVI, Fig. 3, A, B, and C, m and n, Fig. 2, E and F, m and n, Plate XXV, 

 F, O, and H, and so on. 



The first change which takes place is the differentiation of the 

 peripheral cells from the central ones, as shown in Plate XX, Fig. 1 and 

 Fig. 6. As is shown in the series of figures above enumerated, the 

 peripheral cells gradually assume the character of an epithelium, and 

 this change first takes place at the distal end on the haemal surface, and 

 gradually runs backwards and also spreads upwards over the sides of the 

 germinal mass, until at last the central core of cells, n, is shut in by a 

 follicular epithelium, m, as shown in Plate XVI, Fig. 5. In all except very 

 young specimens, the epithelium on the haemal side of the germinal mass 

 extends from end to end, but for a long time the epithelium is absent 

 from the dorsal side, and, even in old stolons, there is an unspecialized 

 remnant of the embryonic germinal mass at the root of the stolon. 



A series of transverse sections across the genital string is shown 

 highly magnified in Plate XXXI, Figs. 4, 5, 6, 7, 8 and 9. They are from 

 an embryo of about the same age as the one shown in Plate XLI, Fig. 5, 

 and the entire stolon is shown in surface-view in Plate XLI, Fig. 6. The 

 stolon is completely formed, but there are as yet no traces of the con- 

 strictions to mark out the bodies of the chain-salpae, although these 

 make their appearance at about this time and are sometimes found at 

 the tips of much younger stolons. 



Near the base of the stolon, Fig. 4, the greater part of the genital 

 string consists of embryonic germ cells, and only those upon the haemal 

 surface nearest the ectodermal wall of the stolon are arranged in an 

 epithelium. On the middle line the epithelium is well marked, and it 

 consists of cells which are quite different from those which make up the 

 rest of the genital string, but at the sides it gradually loses its distinct 



