136 



DEVELOPMENT OF THE HEART. 



muscular wall of the heart. It encloses a second tube composed of flattened 

 epithelium cells ; this so-called endothelM Me (His) becomes the lining epithelium 

 of the endocardium. 



S-rv 



Fig. 161. A, TRANSVERSE SECTION THROUGH THE HEAD OP AN EMBRYO RABBIT OP EIGHT DAYS AND 



FOURTEEN HOURS, WITH A PART OF THE PERIPHERAL BLASTODERM. \ s . (Kolliker. ) 



hh, rudiments of the heart ; sr, pharyngeal groove, with notochordal thickening of hypoblast. 



B. PART OF THE SAME MORE HIGHLY MAGNIFIED. 1 | 2 . (Kolliker.) 



Lettering as in fig. 160. In addition : a/Jt, fold of splanchnopleure to form wall of heart; 

 ihh, endothelial tube of heart. 



There is some doubt as to the source of this endothelial tube of the heart. In the preceding 

 edition of this work it was stated that it is " derived from the deeper part of the visceral rneso- 

 blast ; " this statement being- apparently founded upon the statements and figures given by 

 Kolliker. His ascribes it, like the endothelium of the blood-vessels, to an ingrowth from the 

 vascular area. The appearance of the section shown in fig. 161 B, seems to lend colour to the 

 belief tl'at the invagination which has taken place to form the heart is not the splanchnic 

 mesobiast only, but has included also the hypoblastic layer of the splanchnopleure ; the notch 

 which is seen in the hypoblast near su^ appearing to indicate an interrupted connection with 

 the endocardial tube. Should future investigations show that this is actually the mode of 

 formation of the tube, the mammalian heart would be developed in essentially the same 

 manner as has been shown by Riickert to occur in Pristiurus (an Elasmobranch), where this 

 organ, which, as in all vertebrates below mammals, is formed only after the foregut is com- 

 pleted, is developed as a median outgrowth or thickening of the ventral wall of the foregut. 

 A similar mode of formation has also been noticed in Cyclostomata, Ganoids, and Amphibia. In 

 reptiles and birds the first appearance of the heart is as a bilateral tube, but it becomes visible 

 only after the foregut is formed, and the two tubes lie from the first close together, and from 

 the surface appear as a single median tube. 



Sections at a somewhat later period (fig. 162) show the two tubes lying in con- 

 tact on the ventral side of the now completed foregut. The septum which divides 

 them at this period has nothing whatever to do with the permanent intra-cardiac 

 septum, but soon becomes completely absorbed, so that by the fusion of the two 

 lateral tubes a single median tube is the result (fig. 163). This median tube remains 

 attached by a suspensory membrane resembling a mesentery (mesocardium posterius) 

 (mp, fig. 163) to the ventral wall of the pharynx, but the mesocardium anterius, 

 which also at first results from the fusion, disap"?ars, except at the lower end, and 

 otherwise the tube becomes free, except where the vitelline veins pass to it from the 

 yolk sac, a lateral attachment to the body wall being here subsequently formed on 

 each side (mesocardium laterale of Kolliker). After.it is thus formed, the heart is 

 for a time median in position and symmetrical (fig. 164, A), but already in the 



