250 JOHNS HOPKINS UNIVEESITY MORPHOLOGICAL MONOGRAPHS. 



XII, XIII and XIV, while it remains thick for a very much longer time 

 near and upon the middle line of the dorsal surface of the embryo, as shown 

 at the top of Figs. 5 and 6 of Plate XXII, and still better in the hori- 

 zontal sections, Plate XVII, Fig. 5, &', and Plates XIII and XIV, Figs. 1 

 and 2, b'. The true ectoderm first appears on the middle line underneath 

 this thickened portion of the epithelial capsule, as shown at A in cut D, 

 and very highly magnified, from another section of the same specimen 

 in Plate XLII, Fig. 11, where 21 is the outer and 22 is the inner fold of 

 the embryo sac, b' the cells of the epithelial capsule, and a' the ecto- 

 derm. As the ectoderm grows and spreads over the embryo it at first 

 pushes under the cells of the epithelial capsule and crowds them out and 

 stretches them apart as shown .in the last figure, and in Fig. 9 of the same 

 plate where a' is the ectoderm and b' the cells of the epithelial capsule. 

 As the ectoderm becomes more developed many of these cells are often 

 pushed along before its growing edge and crowded together in an 

 irregular pile, as shown at b' in Plate XVII, Fig. 4 and Fig. 1. The 

 details of the process of moulting vary greatly. In some specimens big 

 scales and flakes of epithelium are dropped off and are found around the 

 embryo, but as a rule the cells separate from each other before they 

 become detached and they drop off separately. The moulted cells are 

 shown from Salpa hexagona at B' in Plate XLV, Fig. 3, and in Salpa 

 pinnata in Plate XVI, Figs. 2 and 3, at b'. Fig. 6 of this plate is a por- 

 tion of the ectoderm from another section of the same embryo very 

 much magnified ; 21 and 22 are the outer and inner folds of the embryo 

 sac ; 25, the brood chamber ; b', the cells of the epithelial capsule ; a, the 

 ectoderm, and 15, the body cavity with amoeboid follicle cells. When 

 the cellulose mantle first makes its appearance it is a thin transparent 

 layer, Plate XLV, Fig. 3, v, in contact with the ectoderm, and carrying 

 on its outer surface the scattered cells, b', of the epithelial capsule. These 

 cells soon drop off, however, and at a stage a little older the outer surface 

 of the cellulose mantle, Plate XLV, Fig. 4, v, is naked, although a cell, 

 apparently from the epithelial capsule, may occasionally be found 

 embedded in it, as shown at v in Plate XVII, Figs. 6 and 7. 



III. The supporting ring. As regards the fate of -that part of the 

 epithelial capsule which I have called the supporting ring of the placenta, 

 23 of the figures, there is a very important difference between Salpa 

 pinnata and those stages of Salpa hexagona which I have studied. In 

 the latter species it is, up to the time of birth at least, simply a 

 thickened ring-like support or pessary for holding the embryo and the 



