82 THE CHANGES FROM STAGE A TO F. 



description of the changes by which the adult body cavity, 

 pericardial cavity, and heart are formed. 



The Body Cavity and Vascular System. 



I have already described the first appearance of the body 

 cavity. It arises in Stage d as a space between the dorsal 

 ectoderm and the endoderm (Plate VI, fig. 13 b. h.), and 

 between the ventral ectoderm and the endoderm [b. be.) , There 

 also appears at the same stage a space in the parietal thickening 

 of the walls of the somites (Plate VI, fig. 13, b.lat.). In 

 later embryos of Stage d (Plate VII, figs. 17a — c?), tliese spaces 

 are all more marked, and cells — apparently amoeboid wan- 

 derers from the walls of the somites — have made their appear- 

 ance in the two former (Plate VII, fig. 17 a — d.). These cells 

 apply themselves to the ectodermal and endodermal walls of the 

 chambers in which they are contained, and so form the founda- 

 tion of the mesodermic investment by which the body cavity 

 of the adult is lined. In the next stage the cavities b. h., b. be, 

 remain unchanged ; but the cavities in the parietal thickenings 

 become definitely established (Plate VII, fig. 21 a — c, b. lat). 

 The latter at this stage appear to be segmentally arranged ; 

 each one beginning at the anterior end of a somite, and extend- 

 ing backwards to the level of the appendage, in the mesoderm 

 thickening of which it is lost. They are bounded internally 

 by the septum which runs from the ventral border of the somites 

 along the inner side of the nerve-cord to the ventral body wall, 

 and externally by a mass of mesoderm cells which project into 

 what I have called the limb-ridge (Plate VII, fig. 31 a, I. r.). 

 In the next stage (Stage e, Plate VII, figs. 23 a — c), the two 

 median cavities, b. h. and b. be, present but little alteration, 

 excepting that the dorsal one b. h. has been encroached upon by 

 the dorsal extension of the median division of the somite. 



The successive lateral spaces have now become continuous, 

 extending through the region of the appendage immediately 

 within the septum (v. s.). This is shown in Plate VII, 



