THE VASCULAR SYSTEM. 619 



the peritoneal linings of the anterior divisions of the body cavity, 

 shut off from the remainder of the body cavity by the diaphragrh. 



''S/>.C. 



At-' 



Fig. 355. Section through an advanced embryo op a Babbit to shew how thh 

 pebicabdial cavity becomes surrounded by the pleural cavities. 

 ht. heart; pc. pericardial cavity; pl.p. pleural cavity; Ig. lung; al. alimentary 

 tract; ao. dorsal aorta; ch. notochord; rp. rib; st. sternum; sp.c. spinal cord. 



The exact mode of formation of the diaphragm is not fully made 

 out; the account of it recently given by Cadiat (No. 491) not being 

 in my opinion completely satisfactory. 



BiBLIOGKAPHY. 



(491) M. Cadiat. "Du d^veloppement de la partis c^phalothoracique de I'em- 

 bryon, de la formation du diaphragme, des pleures, du p^ricarde, du pharynx et de 

 I'cesophage." Journal de VAnatomie et de la Fhysiologie, Vol. xiv. 1878. 



Vascular System. 



The actual observations bearing on the origin of the vasculai- 

 system, using the term to include the lymphatic system, are very 

 scanty. It seems probable, mainly it must be admitted on a priori 

 grounds, that vascular and lymphatic systems have originated from 

 the conversion of indefinite spaces, primitively situated in the general 

 connective tissue, into definite channels. It is quite certain that 

 vascular systems have arisen independently in many types; a very 

 striking case of the kind being the development in certain parasitic 

 Copepoda of a closed system of vessels with a red non-corpusculated 

 blood (E. van Beneden, Heider), not found in any other Crustacea. 

 Parts of vascular systems appear to have arisen in some cases by a 

 canalization of cells. 



