W. K. BROOKS ON THE GENUS SALPA. 213 



Seeliger is disposed to regard the mesenchyma cells as the source of 

 the "mesoderm" of the stolon, but the very small size of their nuclei as 

 compared with those of the germinal mass renders this very doubtful. 

 In fact an entire mesenchyma cell is hardly as large as one of the germ- 

 inal nuclei, as will be seen from the figures. The nuclei of the migratory 

 follicle cells, Plate XLI, Fig. 7, B, are about equal in size to those of the 

 germinal cells, but they are vesicular and transparent, with only a few 

 minute granules of chromatin, and they multiply directly, so that each 

 follicle cell usually contains two, as is shown in the figure, and also in 

 Plate XX, Figs. 5 and 6 ; while the germinal nuclei contain large chro- 

 matin granules, and they multiply by indirect division with nuclear 

 figures. 



I have not been able to trace the actual connection between any of 

 the blastomeres of the embryo and the germ cells, but in the chapter on 

 the germ layers I shall show that the blastomeres are distinguished by 

 features which are found again in the cells of the germinal mass, and 

 there is therefore reason to believe that they are the source from which 

 these latter are derived. 



Plate XX, Fig. 6, shows the youngest germinal mass which I have 

 found, and another section from the same specimen is shown, much more 

 magnified, in Plate XLI, Fig. 7. In both figures / is the pericardium, c is 

 the pharynx, d' is the endoderm, and a' the ectoderm. The germinal mass 

 is a granular, protoblastic body, with faintly-marked cell outlines filled 

 with transparent nuclei a little larger than those of the endoderm and a 

 little smaller than those of the ectoderm. The nucleus has a membrane, 

 and it contains granules of considerable size, which are usually placed 

 around the periphery in such a way as to appear like a dotted or broken 

 outline. 



Cell multiplication goes on rapidly at this stage in all parts of the 

 germinal mass, and two or three nuclear figures may be found in nearly 

 every one of the sections. 



Seeliger believes that his observations on pyrosoma (15) confirm 

 his account of the origin of the germinal mass in salpa, and he says that 

 the space between the ectoderm and the endoderm of the young stolon 

 becomes filled in pyrosoma, as he has described it in salpa, by an unspe- 

 cialized mass of migratory mesoderm cells, from some of which the 

 genital rod is afterwards differentiated, while the rest of them give rise 

 to the nerve-tube and perithoracic tubes and the mesoderm. 



Salensky, however, who has recently made a minute study of the 



