W. K. BROOKS ON THE GENUS SALPA. 73 



SECTION 6. The Blood-Tubes. 



There are two of these, the upper one, j, and the lower one, ?', both 

 colored yellow in the sections. They communicate with each other at 

 the tip of the stolon, around the tip of the endodermal tube. Elsewhere 

 they are separated from each other by the endodermal tube and the 

 perithoracic tubes, which are so placed as to form a horizontal longi- 

 tudinal partition. In their origin the tubes are part of the cavity of the 

 body of the solitary embryo, and they are present in the stolon at all 

 stages. When the ectoderm and endoderm which are to enter into the 

 structure of the stolon first become differentiated from the tissues of the 

 embryo, Plate XX, Fig. 6, these two layers are in contact with each other, 

 and, as Plate XLI, Fig. 7, shows, both the ectoderm, a, and the endoderm, 

 6, are in direct contact with the germinal mass, n, but as soon as the 

 rudimentary stolon begins to elongate, Plate XX, Fig. 7, the endoderm 

 separates a little from the other structures, so that there is space for 

 the blood corpuscles of the solitary salpa to circulate between them, 

 and these spaces are present at all subsequent stages, as the transverse 

 sections in Plates XX and XXI, and the longitudinal section in Plate 

 XVI, Fig. 5, show. 



The lower blood space, , is the first one to be folded into the stolon, 

 as shown in Plate XXI, Figs. 1 and 2, i ; and in Plate XXXV it is contin- 

 uous with that part of the body of the solitary salpa which contains the 

 eleoblast, k. I shall show farther on that the degenerating cells of the 

 eleoblast furnish food for the blood corpuscles and migratory mesoderm 

 cells of the embryo, and as the sections of the stolon show that these 

 latter pass into it in great numbers, they no doubt play an important 

 part in the nutrition of the growing stolon and the chain-salpse which are 

 formed from it. The upper blood space, j, is in intimate relation with the 

 heart, e, and the great ventral blood-vessel of the solitary embryo, and it 

 will be necessary to say a few words here about these structures. The 

 pericardium, / of the sections, and the heart, e, are shown in horizontal 

 sections of the solitary embryo, in Plate XIX, and in vertical section in 

 Plate XXXV. The pericardium is a closed vesicle, colored yellow in the 

 figures, behind the pharynx, c, and to the left of the middle line, and 

 the heart is formed by the infolding of its walls on the side next the 

 pharynx, as shown at e in Plate XIX, Figs. 1, 2, 3, 4 and 5. In its origin 

 the chamber of the heart is thus part of the body cavity, external to the 

 pericardium. In the young embryo it is behind the pharynx, c, and 



