118 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



Plates XXV-XXXI. The structures which are involved in the formation 

 of the body proper have moved down onto the lower surface of the axial 

 tube, and are beginning to approach each other on the middle plane of 

 the body, which lies pretty nearly in the imaginary line between the 

 letter n and the Fig. 27. This is a most important stage, and its careful 

 examination is essential to a comprehension of the development of Salpa 

 pinnata. The course which the ganglion has taken in its migration from 

 its primitive position to the one which it occupies at this stage will be 

 understood from the study of the figures on Plate V. 



In Fig. 1 all the ganglia are on the middle line of the stolon and in a 

 row, as is shown in Plate XV, Fig. 10. In Fig. 2 those which belong to 

 right-hand salpse have moved to the right, and those which belong to 

 left-hand salpae to the left, although they are still near the top of the 

 stolon, as is shown in Plate XXXI. 



In the next stage, Fig. 3, they have begun to move down towards the 

 lower surface of the stolon, although they are still above the level of the 

 endodermal tube^ while in Fig. 4 they have reached the level of the 

 endodermal tube, as is shown in Plate XXX, Fig. 2, at F for a right-hand 

 salpa, and at E' for a left-hand one. Finally, in the stage shown in 

 Plate VI, Fig. 1, they are near the level of the bottom of the stolon, as is 

 shown for a right-hand salpa in Plate XXVIII, Fig. 1, H, and for a 

 left-hand salpa in Plate XXVI, Fig. 2, G'. As the salpa grows older the 

 ganglion moves further and further from its primitive position, as is 

 shown by comparing the figures in Plate VI, 1, 2 and 4, Plate VII, 3, 4 

 and 5, and Plate VIII, Fig. 1. 



Returning now to Plate VI, Fig. 1, it will be noticed that the 

 extension of the two pharyngeal pouches downwards has resulted in the 

 separation of each pouch into a small connecting tube and an expanded 

 end within the body of the salpa. One of these connecting tubes, 27, 

 joins the right pharyngeal pouch to the right side of the endodermal 

 tube, while the other, 28, joins the left one to the left side of the 

 endodermal tube. The latter is the same as the one shown at an earlier 

 stage for a left-hand salpa at 27 in Plate V, Fig. 4, and is also the same 

 as the lower part of tube 28 in Plate VIII, Fig. 2, while the former, 27, 

 is the same as tube 27 of Plate VIII, Fig. 2. 



The three upper sections through Plate VI, Fig. 1, hardly call for 

 explanation, although examination of the sections shows that an impor- 

 tant change, the obliteration of the folds of ectoderm between the salpae, 

 has already begun in this region, and at a later stage the ectoderm of all 



