266 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



The inner end of each perithoracic tube divides into two branches, 

 a horizontal branch which meets and unites with its fellow, on the 

 middle line, to form the atrium, G'" of the cut, and g'" of Plate XI, 

 Figs. 4 and 5, and of Plate XII, Fig. 2, and a vertical branch, G of the 

 cut, g' of Plate XII, Fig. 4, and g of Fig. 5, which ultimately opens as a 

 gill-slit into the pharynx, as shown at G in cut C, and at c in Plate XIV, 

 Fig. 9. After the atrium is formed by the union of the horizontal 

 branches of the perithoracic tubes, these latter lose their external 

 openings and their connection with the somatic layer ; and they move 

 inwards until they lie side by side on the middle line of the body above 

 the cloaca, as shown in G" of cut C, and also in Plate XIII, Fig. 6, g", and 

 in Plate XIV, Fig. 3, g". It will be seen that as the result of these changes 

 a complicated structure composed of follicle cells from the somatic layer 

 is folded into the substance of the visceral mass of follicle cells. 



The various organs of the body of the embryo now become mapped 

 oat in the follicle cells, as is shown by the series of horizontal sections in 

 Plates XII, XIII and XIV ; but as a knowledge of the details of this 

 process is not needed to understand the later history of the follicle, they 

 will not be noticed now. 



The follicle reaches its greatest completeness at the stage which is 

 shown in Plate XII and in cut B, and it will be necessary to examine 

 these figures a little more minutely before we consider the fate of the 

 follicle. The sections are from an embryo of Salpa pinnata a little 

 younger than Fig. 1 of Plate XLI. Cut B, p. 29, is a careful reconstruc- 

 tion from all the sections. Comparison with the cut will show that the 

 first section figured, Plate XVII, Fig. 5, cuts first on the right side the 

 outer fold, 21, of the embryo sac, next the blood space, y, between the 

 outer fold and the inner one, 22, which is separated in fresh specimens 

 from the more central structures by the cavity of the embryo sac, which 

 I have represented in the diagram, although the delicate unsupported 

 folds are shrunken in specimens which have been imbedded in paraffine. 

 Next comes the epithelial capsule B' or &', which is colored red in the 

 figures. It will be noted that in this section the somatic layer of the 

 follicle and the visceral layer are in contact with each other along the 

 middle line, on what later stages show to be the dorsal surface of 

 the embryo. The section next cuts the right half of the follicle above 

 the level of the body cavity ; next the ectodermal blastomeres, A', then 

 the left half of the follicle and the top of the body cavity, and so on. 

 The second section which is figured, Plate XII, Fig. 1 and cut B, passes 



