THE CtELOM. 427 



DEVELOPMENT OF THE CCELOM. 



The coelom, or body-cavity, of the rabbit appears, as in most 

 Vertebrates, as a cleft in the mesoblast, formed by splitting, or 

 rather by rearrangement, of its cells into two layers, somatic and 

 splanchnic. 



The coeloun appears first on the eighth day, and by the ninth 

 day (Fig. 1 16, c) has become a cavity of considerable size. It 

 is not confined to the embryo, but stretches out beyond this, 

 and in all directions, reaching almost to the margin of the 

 mesoblast, indicated by the sinus terminalis, Si. 



Immediately in front of the embryo, in the pro-amnion 

 (Fig. 145, AN 1 ), there is at first no mesoblast, and consequently 

 no coelom ; but in the later stages, as the mesoblast invades the 

 proamnion from its sides, the coelomic cavity extends into this 

 region also. 



Within the embryo itself, the coelom is confined to the body 

 region, and does not extend forwards into the head. The 

 abdominal portion of the coelom presents no further changes of 

 special interest, but in the thorax the development of the 

 pericardial and pleural cavities, and also the formation of the 

 diaphragm, require notice. 



1 . The Pericardial Cavity. 



Early on the ninth day, the heart consists of two tubes, 

 lying along the sides of the head, and widely separate from each 

 other (Fig. 145, R). The parts of the ccelom into which these 

 tubes project become later on the pericardial cavity, so that this 

 cavity, like the heart itself, consists at first of two separate 

 halves, right and left respectively. 



As the side-folds deepen, the two halves of the heart are 

 brought together beneath the pharynx ; and, early on the tenth 

 day, the right and left halves of the pericardial cavity meet 

 beneath the throat, and become continuous with each other. 



The pericardial cavity (Fig. 147) is thus merely the anterior 

 part of the general body-cavity or coelom, and there is at first no 

 boundary between the two, except the very imperfect partitions 

 formed by the right and left vitelline veins, where they diverge 

 behind the heart. 



Towards the close of the tenth day, and during the early 



