288 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



This is not literally true, for the ectoderm itself takes a most active part 

 in the formation of the stolon, as I have shown on p. 68. This is marked 

 off on the body of the embryo by a fold of ectoderm which pushes back- 

 wards from its tip to its base, so that it is folded off from the body of 

 the embryo rather than pushed out, and in the younger stages of Salpa 

 pinnata at least, its differentiation from the body of the embryo is chiefly 

 due to the active growth of this ectodermal fold, as I have shown on p. 68 

 and in Plate XX, Figs. 5, 6 and 7, Plate XLI, Figs. 4 and 6, and Plate 

 XVI, Fig. 5. 



The ectoderm of the aggregated salpa has been correctly held to be 

 directly derived from the ectodermal tube of the stolon by all students 

 except Todarro. My own observations also show that the multiplication 

 of the ectoderm cells is the chief active agent in the segmentation of 

 the stolon, as I have stated on p. 78. This is exhibited by the following 

 series of figures, Plate XXXIV, Figs. 8, 9, 10, 11, Plate XXIV, Fig. 5, 

 Plate XV, Figs. 11 and 12, and Plate XXIX, Figs. 3, 4, 5, 6, 7 and 8. 



These figures show that the nerve tube and the perithoracic tubes 

 are cut up into vesicles by the growth of the ectodermal folds, and that 

 these are the chief agents in the segmentation of the endodermal tube 

 and of the genital string. 



SECTION 5. TJie Nervous System of the Salpa Embryo. 



The salpa embryo is so very unfavorable for minute study that we 

 cannot hope to do more than to show that the history of its organs is not 

 inconsistent with our knowledge of the other tunicates. We must not 

 look to it for contributions to comparative embryology, and I have little 

 to add to the outline of the history of its nervous system which I have 

 given on p. 37 and p. 41. 



At the stage which is shown in Plate XII and in cut B on p. 29, the 

 middle of the dorsal surface is occupied by the ectodermal blastomeres, A', 

 and at each end of the body these may be traced downwards into the 

 visceral mass. 



Those at the posterior end are shown at 16 in Plate XII, Fig. 1, at 9 

 in Fig. 2, and at 18 in Figs. 3, 4 and 5. I regard them as the blastomeres 

 of the caudal portion of the nervous system. They are inclosed in a 

 mass of follicle cells which lies behind a second mass of blastomeres and 

 follicle cells, Fig. 4, 19, which I regard as the rudiment of the notochord. 

 The blastomeres of both these regions soon become undistinguishable, 



