406 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



The skeleton of the limbs develops so far as is known as in 

 Birds, from a continuous mesoblastic blastema, within which the 

 corresponding cartilaginous elements of the limbs become dif- 

 ferentiated. 



The body cavity. The development of the body 

 cavity and its subsequent division into pericardia! 

 pleural and peritoneal cavities is precisely the same in 

 Mammalia as in Aves (p. 264 et seq.). But in Mam- 

 malia a further change takes place, in that by the for- 

 mation of a vertical partition across the body cavity, 

 known as the diaphragm, the pleural cavities, contain- 

 ing the lungs, become isolated from the remainder of 

 the body or peritoneal cavity. As shewn by their 

 development the so-called pleurae or pleural sacs are 

 simply the peritoneal linings of the anterior divisions 

 of the body cavity, shut off from the remainder of the 

 body cavity by the diaphragm. 



The vascular system. 



The heart. The two tubes out of which the heart 

 is formed appear at the sides of the cephalic plates, 

 opposite the region of the mid- and hind-brain (Fig. 

 107). They arise at a time when the lateral folds 

 which form the ventral wall of the throat are only just 

 becoming visible. Each half of the heart originates in 

 the same way as in the chick ; and the layer of the 

 splanchnic mesoblast, which forms the muscular wall for 

 each part (ahh). has at first the form of a half tube open 

 below to the hypoblast. 



On the formation of the lateral folds of the splanchnic 

 walls, the two halves of the heart become carried inwards 



