DEVELOPMENT OF THE MUSCLES. 159 



DEVELOPMENT OP THE SEROUS CAVITIES AND OF THE 

 MUSCLES AND SKELETON. 



The serous cavities peritoneum, pleuras, pericardium are derived from the 

 original split or cleavage of the mesoblast, which constitutes the ccelom or general 

 body cavity (pleuro-peritoneal cavity of older authors). This cleft is formed in the 

 head as well as in the trunk, and when the heart is formed as a double tube, each 

 half is enclosed within a portion of that cavity, which later on, when the body walls 

 bend round and meet to enclose the fore-gut, comes, like the heart itself, to occupy a 

 position on the ventral aspect of the alimentary tube. The part of the ccelom which 

 thus contains the heart is not for some time entirely distinct, but communicates 

 dorsally by two comparatively narrow channels with the anterior part of the general 

 body cavity, here separated into lateral halves, which ultimately become the pleurae, 1 

 by the alimentary canal. Subsequently these communications become obliterated, 

 and the heart-ccelom separated as a distinct cavity (pericardial cavity). Below, 

 where the great veins enter the heart, they pass into a mass of mesoblastic tissue, 

 which is connected with the anterior body wall (where it receives the umbilical 

 and vitelline veins), and also with the lateral wall (where it receives the ducts of 

 Cuvier), and which, as the heart bends, so that the venous end passes behind the 

 ventricle and bulb, is carried along with the veins up behind that organ, and thus 

 forms an obliquely placed thick septum, at first incomplete, but subsequently becoming 

 entirely closed, which separates the heart within the pericardial part of the body 

 cavity in front and above from the stomach and alimentary canal within the peri- 

 toneal part of that cavity behind and below. The thick septum, besides containing 

 the saccus reuniens and the portions of the great veins (vitelline, umbilical, ducts of 

 Cuvier) which open into that cavity, also contains the rudiments of a part of the 

 diaphragm and the mesoblastic part of the liver, into which the hypoblastic part 

 grows from the adjacent duodenum ; it has been termed by His the transverse septum 

 (see figs. 177, 178). As development proceeds, the septum becomes gradually diffe- 

 rentiated into its several parts. The great veins become still further shifted behind 

 the heart, and the saccus reuniens becomes incorporated with that organ. The 

 liver, which is at first contained entirely within the septum, becomes split off from 

 its upper layer, which now forms the thin portion of the diaphragm, while the cavity 

 of the peritoneum extends from either side, and separates them from one another, 

 except along the attachment of the broad ligaments. 



The diaphragm is completed by a growth of mesoblast which occurs on each 

 side, and cuts off the anterio -dorsal portions of the body-cavity into which 

 the lungs are invaginated (recessus pulmonales) from the posterior or peritoneal 

 part. 



The serous membranes are formed by differentiation of the lining mesoblast of 

 the coelom. 



The formation of the omenta, and the changes which the mesenteric folds of 

 peritoneum undergo, have been already mentioned in connection with the develop- 

 ment of the abdominal viscera. 



Development of the muscles. The muscles of the trunk are formed from 

 the protovertebrse. These are at first, as previously described, separate masses of 

 mesoblast, the cells of which have at the periphery of the mass a tendency to a 

 radial disposition (fig. 189, A), whilst toward the centre they are loosely arranged, 



1 The manner in which the pleura are invaginated by the growing lungs has already been alluded to 

 (p. 110). 



