22 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



closely wrapped around it, forms the covering which I shall call the 

 epithelial capsule of the embryo. The way in which the growing embryo 

 comes to project into the cloaca is also shown in Plate IX, Figs. 1 to 9, 

 where the blood-space, y, is colored yellow, and the cavity, c, of the 

 cloaca red. 



The epithelial capsule covers the embryo during the early stages of 

 development, and it is shown at b' in Plates XI, XII, XIII, XIV and 

 XXII, and in cuts B, C and D. As it does not grow, it becomes distended 

 by the growth of the embryo, and its cells grow more and more flat and 

 farther and farther apart; and as the ectoderm is formed under it, it 

 breaks up into separate cells which are thrown off as shown at 6' in Plate 

 XVI, Figs. 2 and 3, and in Plate XVII, Fig. 4, and at B' in Plate XLV, 

 Fig. 3. Plate XVI, Fig. 6, is a part of Fig. 2 very highly magnified to 

 show the formation of the ectoderm and the molting of epithelial cap- 

 sule. In this figure, 15 is the body cavity of the embryo, a its ectoderm, 

 and b' the detached cells of the epithelial capsule; 21 and 22 are the outer 

 and inner folds of the embryo sac, which is to be described soon. 



It will be seen that, at the stage shown in Plate XI, Fig. 3, that part 

 of the embryo which is at the bottom is not covered by the epithelial 

 capsule, but is directly exposed to the blood of the chain-salpa which 

 circulates in the space y. 



While this uncovered area subsequently becomes smaller, as compared 

 with the growing embryo, it never becomes covered in completely, and 

 within it the placenta is formed. This organ, which serves to nourish 

 the growing embryo with food derived from the blood, is shown in Plate 

 III, Fig. 4, pi, and in Plate XLI, Figs. 1, 2, 3 and 5, pi. It is also shown 

 in longitudinal section in Plate XXXV at y". It preserves its communi- 

 cation with the blood-channels of the chain-salpa until the embryo is 

 born, and as this is nourished from the blood which passes into and out 

 of the placenta, its function and its anatomical relations are strikingly 

 like those of the mammalian placenta, although there is a very important 

 difference which will be described soon. 



The fold in the wall of the cloaca which covers the embryo and forms 

 the epithelial capsule, soon extends down for some distance below the 

 level of the embryo, as is shown in Plate XLV, Fig. 1, and forms the 

 boundary of a spacious chamber, the cavity of the placenta, which opens 

 through a constricted neck into the blood-channel of the chain-salpa. 



The cells which compose this wall soon become elongated and 

 thickened, as is shown in the figure at 23, while those which cover the 



