THE VASCULAR SYSTEM. 



635 



of the mesoblast by protoplasmic processes. A second layer 

 next becomes split from the splanchnic mesoblast, connected 

 with the first layer by the above-mentioned protoplasmic 

 processes. These two layers form together the epithelioid lining 

 of the heart ; between them is the cavity of the heart, which soon 

 loses the protoplasmic trabeculae which at first traverse it. The 

 cavity of the heart may thus be described as being formed by a 

 hollowing out of the splanchnic mesoblast, and resembles in its 

 mode of origin that of other large vascular trunks. 



The above description applies only to the development of 

 the heart in those types in which it is formed at a period after 

 the throat has become a closed tube (Elasmobranchii, Amphibia, 

 Cyclostomata, Ganoids (?)). In a number of other cases, in 

 which the heart is formed before the conversion of the throat 

 into a closed tube, of which the most notable is that of Mammals 

 (Hensen, Gotte, Kolliker), the heart arises as two independent 



A. 



B. 



mes fir 



FIG. 358. TRANSVERSE SECTION THROUGH THE HEAD OF A RABBIT OF THE 



SAME AGE AS FIG. 144 B. (From Kolliker.) 

 B is a more highly magnified representation of part of A. 



rf. medullary groove; mp. medullary plate; riv. medullary fold; h. epiblast ; 

 dd. hypoblast; dd' . notochordal thickening of hypoblast; sp. undivided mesoblast; 

 ^.somatic mesoblast; dfp. splanchnic mesoblast; ph. pericardial section of body 

 cavity; ahh. muscular wall of heart; ihh. epithelioid layer of heart; vies, lateral 

 undivided mesoblast ; sw. part of the hypoblast which will form the ventral wall of 

 the pharynx. 



