212 JOHNS HOPKINS UNIVEESITY MORPHOLOGICAL MONOGRAPHS. 



each other, and it is a very small object, while the stolon of Salpa pinnata 

 is much larger. I have found that even with the insight which I had 

 gained from the study of the straight stolons of Salpa pinnata and Salpa 

 cylindrica, the twisted stolons of Salpa cordiformis and Salpa demo- 

 cratica are very puzzling, and while I am thus in the position to recog- 

 nize the industry which Seeliger has displayed, and the faithfulness with 

 which he has tried to overcome the difficulties, I am sure that the more 

 favorable nature of my material has enabled me to reach more trust- 

 worthy results. 



The first point of difference is the time when the germinal mass 

 becomes differentiated from the other structure. 



The young stolon, Plate XXI, Fig. 7, contains a central tube of endo- 

 derm, d', within an outer tube of ectoderm, a'. Seeliger says (page 16) 

 that at a very early stage all the space between these two tubes, that is, 

 all the body cavity of the stolon, is filled by an undifferentiated mass of 

 mesoderm, which soon divides into a ventral genital string, n of my 

 figures, a dorsal nerve tube, t, and two lateral side strings, g and h. 



I find, however, that the germinal mass is sharply separated, at all 

 stages, from the other organs of the stolon, as will be seen by comparing 

 the figures in Plate XXI with the younger stage shown in Plate XX, 

 Figs. 1, 2, 3 and 7. At a still younger stage, Plate XX, Fig. 6, the germ- 

 inal mass, which is shown more highly magnified in Plate XLI, Fig. 7, 

 lies in the body cavity of the embryo, and, before the tubes of the stolon 

 are formed, it is sharply limited by a distinct boundary, which is not lost 

 at any subsequent stage. 



Instead of arising, as Seeliger says, within the stolon, it is present 

 before the stolon is formed, and it has nothing in common with the nerve 

 tube or the lateral tubes, as I shall show in the chapter on the stolon. 



I have studied sections of each stage in the three rectangular planes 

 of symmetry, and I appeal, with all confidence, to future observers for 

 the confirmation of my statement. 



As regards its origin also, I must take issue with Seeliger, who says 

 that the "mesoderm" of the stolon is formed by free cells which migrate 

 into it, as independent cells, from the body cavity of the embryo. 



As shown in Plate XVII, Fig. 3, and Plate XVIII, Fig. 6, and also in 

 Plate XX, Figs. 5 and 6, the body cavity of the embryo is filled by a 

 complicated and confused mass of cells, among which two types pre- 

 dominate. These are small, amoeboid blood corpuscles, or mesenchyma 

 cells, Plate XLI, Fig. 7, A, and the migratory follicle cells B. 



