DEVELOPMENT OF THE HEART. 



135 



The mesoblastic cleft (coelom, p/i) is somewhat dilated, but is not doubled in, as in the 

 heart region. The lateral mesoblast ceases a short distance beyond it. Fig. 161 is 

 a section through the middle of the head region of the same embryo. Here, while 



Fig. 158. EMBRYO RABBIT OP EIGHT DAYS AND EIGHTEEN HOURS, WITH 9 PROTOVERTEBILE, VIEWED 



FROM THE VENTRAL ASPECT. 2 (Kolliker. ) 



Fig. 159. SKETCHES SHOWING MORE ADVANCED CONDITION OP THE BITUBULAR HEART OP THE RABBIT. 



-f. (Allen Thomson.) 



A, view from below of an embryo in which the formation of the heart was somewhat more advanced 

 than in fig. 158, and of which an outline of the heart is repeated in B. C, from another embryo, shows 

 the two halves of the heart in the commencement of their coalescence, h, the part of the bent tube 

 which becomes the ventricle ; a, primitive aortic arches and descending aortaa ; VV, vitelline veins 

 entering the heart posteriorly The arrows indicate the course of the blood. 



the other parts of the section are much the same as in front, the dilatation of the 

 ccelom, which is in fact the rudiment of the future pericardium, is occupied by an 



Fig. 160. SECTION FROM THE SAME EMBRYO FURTHER FORWARD THAN THAT SHOWN IN THE PRECEDING- 

 FIGURE. (Kolliker.) 



p, paraxial mesoblast ; rf, medullary groove ; r, ridge bounding groove ; mp, medullary plate 

 of hind brain ; h, epiblast ; hp, somatopleure ; dfp, splanchnopleura ; ph, anterior part of coelom ; 

 mes, mesoblast beyond the coelom ; dd, hypoblast ; dd', notochordal thickening ; sw, lateral wall of the 

 developing pharynx. 



invagination, or fold, of the splanchnic mesoblast. This fold becomes subsequently 

 entirely separated and the aperture or line of imagination closed ; it forms the 



