EXPLANATION OF PLATES. 383 



embryo, c, cavity of pharynx of embryo, d, endostyle of embryo, d', endodermal 

 tube of stolon, e, heart of embryo. /, pericardium, g, right perithoracic tube of 

 stolon, h, left perithoracic tube of stolon. I, ganglion, m, follicle, n, ovary. 



PLATE XXI. 

 A SERIES OF TRANSVERSE SECTIONS OF A YOUNG STOLON OF SALPA PINNATA, AT 



ABOUT THE STAGE SHOWN IN PLATE XLI, FlGS. 5 AND 6. 



All the figures were drawn with a Bausch and Lomb i and A eyepiece, and are 

 reduced one-half in photolithographing. 



Figure 1 is nearest the root or proximal end of the stolon, and is the first section 

 in the series ; Fig. 2 is the seventh ; Fig. 3 is the eleventh ; Fig. 4 is the fourteenth ; 

 Fig. 5 is the seventeenth ; Fig. 6 is the twentieth, and Fig. 7 is the twenty-ninth. 



a, ectoderm of embryo, a', ectoderm of stolon, a", reflected layer of ectoderm. 

 b, endoderm of pharynx of embryo, c, cavity of pharynx, d, endostyle of embryo, 

 d', endodermal tube of stolon, e, heart of embryo. /, pericardium of embryo, g, right 

 perithoracic tube of stolon, h, left perithoracic tube of stolon. *, blood space between 

 the endodermal tube and the genital string of the stolon. In the plane of sections 1 

 and 2 this communicates with the eleoblast, k, of the embryo, j, blood space between 

 the endodermal tube and the nerve tube of the stolon. In the planes of Figs. 3 and 4 

 this communicates with the heart, e, of the embryo, k, eleoblast. m, follicle, n, 

 ovary. 



PLATE XXII. 



TRANSVERSE SECTIONS THROUGH AN EMBRYO OF SALPA PINNATA, AT THE STAGE 



SHOWN IN PLATE XIV. 



While the sections in this plate are symmetrical, they are not quite perpendicular 

 to the plane of the horizontal sections in Plate XIV, but parallel to a plane which is 

 inclined forwards, so that they make an acute angle in front and an obtuse angle 

 behind with the plane of Plate XIV. Cut D, on page 31, is a perpendicular transverse 

 section at this stage. 



a", ectoderm rudiment, b', epithelial capsule, c, cavity of pharynx, g', peri- 

 thoracic tubes, g", remains of spiracles, g'", cloaca, s, ganglion, y, blood space 

 of chain-salpa. y', space between the folds of the embryo sac. y", cavity of placenta. 



7, somatic layer of follicle. 8, visceral layer of follicle. JO, placental portion of 

 follicle. 21, outer fold of embryo sac. 22, inner fold of embryo sac. 23, supporting 

 ring. 24, blood bud. 25, brood chamber. 26, migrating follicle cells. 



PLATES XXIII AND XXIV. 



In these plates, and in all the figures of sections parallel to the long axis of the 

 stolon, the proximal surface is at the bottom, and the right side on the right of the 

 figure. Figures 1, 2, 3 and 4 of Plate XXIII, and 1, 2, 3, 4 and 5, XXIV, are from a 

 series of horizontal sections of a stolon of Salpa pinnata, at the stage which is shown 

 in cut N on page 81. 



The two pharyngeal pouches have reached the oral end of the body, and the 

 salpse are more completely marked out in the walls of the stolon than they were at an 

 earlier stage. 



Figure 1 of Plate XXIII is the fourth section in the series ; 2 is the sixth ; 3, the 

 eighth, and 4, the thirteenth. In Plate XXIV, Fig. 1 is the thirty-fourth ; 2, the 

 thirty-seventh ; 3, the forty-second : 4, the fifty-second, and 5, the sixty-second. 



