DEVELOPMENT OF THE CIRCULATORY SYSTEM. 



243 



lines the coelom is a derivative of the hypoblast, and that 

 the coelom was originally a series of segmental diverticula 

 derived from inflections of the hypoblast ; yet, from a clinical 

 point of view, it is better to regard it as part of the lymphatic 

 system, and really a very extensive lymph space. It allows 

 the heart, lungs and abdominal viscera to undergo movements 

 with a minimum of friction. 



rt. d. of Cuuier^ 

 rt. pleura 

 lung bud- 

 dorsal mesogast 



dorsal meso-card 



to left pleura 

 pericardium 



liner bud 



septum transversum 



eck of yolk sac. 



umbilical diuertic. 



becomes tunica uaginalis 



Pig 201 -The Form of the Coelom in a 3rd week Embryo as viewed from the right 



side. 



As may be seen from Fig. 72, page 93, the coelom at first 

 extends beyond the embryo, between the layers of the somato- 

 pleure, which go to form the foetal membranes. With the union 

 of the right and left layers of the somatopleure at the umbilicus 

 part of the blastodermic coelom is shut within the body of 

 the embryo— the intra-embryonic part of the coelom. It is 

 out of this part that the pericardium, pleurae, peritoneum and 

 tunioae vaginales are formed. The coelom commences in front 



