68 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



SECTION 2. The Orientation of the Stolon. 



As the regions of the stolon undergo very great changes of position 

 during its conversion into a series of salpae, it is necessary to fix upon 

 definite terms for its various regions. I shall call the tip of the stolon 

 the distal end. Morphologically this end is anterior, and the end where 

 it joins the body of the solitary salpa is posterior; but as in many 

 species the stolon is not straight, but coiled, the terms distal and 

 proximal are better, and I shall call the root of the stolon its proximal 

 end. As the region along which the nerve tube lies is above, or towards 

 the body of the solitary salpa, I shall call it the top of the stolon, and the 

 opposite surface, where the genital rod lies, I shall call the bottom. The 

 side which is on the right, when the solitary salpa is pictured as placed 

 in front of the reader with its oral end above and its dorsal surface 

 towards the reader, I shall call the right side of the stolon. 



SECTION 3. The Ectoderm of the Stolon. 



This, like the ectoderm of the solitary salpa? and chain-salpae, is 

 colored purple in all the sections. 



As Plate XVI, Fig. 5, and other figures show, it is continuous at the 

 proximal end of the stolon with the ectoderm of the solitary salpa. 



The multiplication of the ectoderm cells is the most efficient agent 

 in the production of the tubular stolon. In Plate XLI, Fig. 2, x marks 

 the place where the stolon is to be developed, although there is as yet no 

 trace of it. Plate XX, Fig. 6, is a median longitudinal section through 

 this region of the body of an embryo a little older, and the youngest 

 in which I have found traces of the stolon. Plate XXXV will serve 

 to show what part of the body is included in this figure, although it is 

 from an embryo younger than Plate XXXV. In Plate XX, Fig. 6, /, 

 colored yellow, is the pericardium ; k is the eleoblast ; n is the germinal 

 mass, or aggregation of embryonic germ cells, in the body cavity of the 

 solitary salpa embryo ; a is the unmodified ectoderm of the embryo, and 

 a' the tract of ectoderm which is to become the ectoderrnal tube of the 

 stolon. The ordinary ectoderm cells of this region of the body are flat, 

 but those which form this tract are cylindrical, and while none in active 

 multiplication are shown in this figure, other sections show that they do 

 multiply, and the ectodermal area of the stolon thus extends farther and 

 farther backwards, as shown at a' in Figs. 5 and 7, until the germinal 

 mass, Fig. 7, n, and the endodermal tube, d', are shut in, or covered by a 



