^ PRIMITIVE FORM OF THE HEART. 785 



vessels, one of which arches over each side of the primitive pharynx 

 and turns backwards below the proto-vertebrge to form one of the two 

 primitive aortic tubes. From each of these last the omphalo-mesenteric 

 arteries pass off into the vascular area. 



According to recent observations by KoUiker and by Hensen (loc. cit.) a still 

 earlier condition of the heart has been perceived in the embryo of mammals, in 

 which there are two separate tubes hollowed out of the lateral parts of the 

 cephalic fold. Each of these tubes is connected with a vein or entering vessel 

 posteriorly, and an artery or out-going vessel anteriorly : these slowly come to- 

 gether and unite by fusion in the middle, in a limited space at first, and then 

 more and more till the single tubular heart results. Each tube is in relation with 

 the isleui-operitoneal cavity of its own side, and when the median fusion takes 

 place the union of these two becomes the pericardium. 



Fig. 584. 



r.So. 



Fig. 584. — Diagrammatic longitcdixal section through the Axis op an Embrto. 



The section is supposed to be made at a time when the head-fold has commenced, but 

 the tail-fold has not yet appeared. A, epiblast ; B, mesoblast ; C, hypoblast ; FSo, fold 

 of the somatopleure ; Sp, and FiSp, fold of the splanclmopleure ; Avi, commencing (head) 

 fold of the amnion ; NO, neural canal, closed in front, but still open behind ; C/i, noto- 

 chord, in front uncleft mesoblast in the ba«e of the cranium ; D, the commencing foregut, 

 or alimentary canal ; Ht, heai-t ; jU^, pleuro-i>eritoneal cavity. 



The rudimental heart in the form now described, exists in the chick 

 at the thirty-sixth hour of incubation, and already, while still consisting 

 of formative cells not differing greatly from those composing the other 

 parts of the mesoblast, begins to exhibit motions of alternating systole 

 and diastole, by slow contractions which begin behind and pass forward 

 to the anterior extremity of the tube ; and a small quantity of imper- 

 fectly formed blood is propelled through the cavity. 



The elongation which the tubular heart now undergoes causes it to 

 lose the symmetrical form ; and its middle part now becomes detached 

 from the lower side of the alimentary canal, and projects downwards (or 

 forwards in the body) with an inclination to the right side of the 

 embryo. 



The heart is now found to be surrounded on the ventral aspect by a 

 median cavity, which is a part of the pleuro-peritoneal space inter- 

 vening between the wall of the heart as splanchno-pleure, and the 

 somato-pleure forming the thoracic wall. This cavity becomes the peri- 

 cardial sac. 



As the development of the tubular heart progresses, the bend 



VOL. II. 3 E 



