522 



TEE HEART. 



which soon loses the protoplasmic trabeculse 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, Cyclo- 

 stomata., 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 (Henseu, Gotte, 

 Kolliker), the heart arises as two independent tubes (fig. 358), which 

 eventually coalesce into an unpaired structure. 



Iti Mammals 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. 358). 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 the whole heart in Elasmobranchii, 

 etc. ; 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 and downwards, and eventually 



^ihh \fi^f>' 



Fig. 358. Tbansvekse section theodgh the head of a Babbit of the same 

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

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

 r/. medullary groove ; mji. medullary plate ; rw. medullary fold ; h, epiblast ; 

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

 /ip. somatic mesoblast; dfp. splanchnic mesoblast; jj/j. pericardial section of body- 

 cavity ; ahh. muscular wall of heart ; i]ch. epithelioid layer of heart ; mes. lateral 

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

 the pharynx. 



