92 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



Fig. 3 shows the details of a part of the right side of Fig. 1 more highly 

 magnified, but it does not require explanation, since comparison with 

 Fig. 1 will be enough. 



As already stated, the first trace which I have found of the strobiliza- 

 tion of the stolon into a chain of salpae is the undulatory outline of the 

 ectoderm, which is shown on the left of Figs. 5 and 6. The peculiar 

 arrangement of the cells seems to indicate that each of the vertical 

 ridges, which are cut transversely by this longitudinal section of the 

 stolon, may be produced by the multiplication of a single row of ecto- 

 derm cells, but I have not been able to prove this. In the stolon from 

 which the sections were made the ridges appear on the left side before 

 they do on the right, but the difference is very slight and is perhaps 

 accidental. They first appear in the ectoderm of the middle region of 

 the stolon, and gradually extend up and down towards its neural and 

 germinal surfaces. 



Figures 8 and 9 are more highly magnified sections through the left 

 half of the middle region of a slightly more advanced part of the same 

 stolon, and 10 and 11 are through the right half. They are longitudinal 

 sections like 4, 5 and 6, and are, of course, at right angles to the section 

 shown in Fig. 3. They show that the ectodermal ridges are from the 

 first almost exactly equal to each other in width, and that between the 

 ridges the ectoderm grows inwards towards the axis of the stolon, in 

 double folds, which thus form deep vertical furrows separating the 

 ridges from each other. As these folds grow inwards they press upon 

 the right and left perithoracic tubes, h and g, in such a way as to con- 

 strict them, and at last to cut them up into series of closed cloacal 

 vesicles, which soon become completely separated from each other, as is 

 shown in the lower part of Fig. 11 at h. My sections indicate that the 

 perithoracic tubes are passive, and that the active agency which divides 

 them up into vesicles is the growth of the ectodermal folds, which, after 

 passing across the perithoracic tubes, begin to push their way in to the 

 lateral masses of thickened endodermal epithelium, as is shown in Fig. 

 11 and, further advanced, in Plate XXIII, Figs. 7 and 8. The ingrowth 

 of the ectodermal folds goes on, carrying the endoderm before it, as 

 shown in Fig. 9 of Plate XXIII, until the lateral masses of endoderm 

 are folded into a series of vertical pockets, Fig. 9, 28, which open into 

 the endodermal tube of the stolon, d'. The endodermal pouch on the 

 right side of the stolon, Plate XXIII, Fig. 8, 27, is the rudiment of 

 the right half of the branchial sac of the chain-salpa ; and that on the 



