76 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



at this point. A horizontal longitudinal section through a very young 

 stolon would probably furnish more decisive evidence, but all my own 

 specimens of the proper stage were used for other purposes. 



SECTION 8. The Mesoderm of the Stolon. 



In addition to the blood corpuscle and the cells which form the walls 

 of the blood spaces, a number of cells become shut into the stolon on each 

 side, between the ectoderm and the perithoracic tube, as shown in Plate 

 XXI, and in Plate XXXIV, Fig. 3, while others are found, as Plate 

 XXXIV, Fig. 3 shows, between the endoderm and the perithoracic tube. 

 Some of these cells appear to enter the stolon as detached mesoderm cells, 

 while it is possible that others are derived from the pericardium, for this 

 unquestionably enters into the structure of the stolon. On the left side 

 of the inverted section in Plate XX, Fig. 1, it will be seen that a diverti- 

 culum from the pericardium, /, pushes in between the ectoderm and the 

 right side of the endodermal tube, and Figs. 4 and 5 of Plate XXI show 

 that the end of this process is constricted by the growing ectoderm, and a 

 part of it, at least, probably becomes included in the stolon, although at 

 an older stage there is nothing which can be identified as its derivative, 

 and if it is represented at all it is probably represented only by inde- 

 pendent cells. 



Another constituent of the stolon may be mentioned here, the gela- 

 tinous substance which fills the spaces in the angles between the endo- 

 thelium of the blood spaces and the other organs. This substance, which 

 is homogeneous and transparent, is shown at 32 in Plate XXXIV, Figs. 

 3 and 5. 



SECTION 9. The Genital Rod. 



The origin and history of the genital rod will be treated at length in 

 another chapter. 



We must note here, however, that its rudiment, Plate XLI, Fig. 7, 

 is present in the body cavity of the embryo before the stolon is formed, 

 and that it is shut into the stolon, as the figures on Plate XX show, by 

 the growth of the ectoderm. 



ft 



SECTION 10. The Derivatives from the Parts of the Stolon. 



The ectoderm of the stolon gives rise to the ectoderm of the chain- 

 salpse, to the organs by which they are fastened to each other after birth, 



