44 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



At the stage shown in Plate XII, the somatic follicle on and near 

 the middle line, just above the placenta, at 7, in Figs. 4 and 5, begins to 

 lose its cell outlines and to become thickened and indefinite, and, as is 

 shown at 7 in Plate XIII, Fig. 9, it very soon breaks up into its con- 

 stituent cells, which separate from each other and become amoeboid, 

 although most of them remain in the places which they occupied at 

 an earlier stage. This separation of the cells is accompanied by the 

 formation of a transparent gelatinous substance between them, so that 

 they become converted into a cartilage-like substance which lies around 

 the circumference of the body cavity of the embryo just above the 

 placenta, and most abundant near the middle axis, as is shown at fc in 

 Plate XVIII, Figs. 1 and 8, and also in Plate XXXV. The disintegration 

 of the somatic layer gradually extends upwards, as is shown in Plates 

 XIII and XIV and in cuts C and D, but as the cells break apart and 

 become free they wander into all parts of the body cavity, and all traces 

 of the somatic layer as a distinct tissue soon disappear, although, as 

 shown in Plate XIII, Figs. 3, 4, 5 and 6, its outline is preserved for a 

 short time after the cells are completely separated. The body cavity, 

 which appears to be empty, is undoubtedly filled with a fluid or semi-fluid 

 serum, of greater density than water, and sufficiently firm to hold the 

 amoeboid cells and to keep them floating. By the time that the pharynx 

 appears, Plate XIV and Plate XXII, the somatic layer is completely 

 broken down. Plate XLII, Fig. 11, is part of a transverse section across 

 the middle line of the dorsal surface of an embryo at the time when 

 the last remnant of the somatic layer is going to pieces, and it shows 

 the way this takes place. In this figure 21 and 22 are the folds of the 

 embryo sac, &' is the epithelial capsule, a is the ectodermal rudiment, 15 

 is the body cavity, and 8 the follicle cells. These are multiplying rapidly 

 by direct division of the nuclei, and their exposed ends are irregular and 

 amoeboid. Many of the cells break away bodily, while others divide into 

 a portion which retains its position, and a portion which splits off as a 

 free amoeboid cell. Multiplication of the nuclei goes on in the detached 

 cells, as well as in those which are fixed, and two nuclei are usually 

 present in most of the wandering cells. 



Thus the embryonic portion of the somatic layer of follicle cells dis- 

 appears as a distinct layer, and at the stage shown in Plates XVI and 

 XVII, and in Fig. 2 of Plate XLI, it is represented only by migratory 

 amoeboid cells scattered through the body cavity of the embryo. 



