82 A HUMAN EMBRYO AT THE BEGINNING OF SEGMENTATION, 



Into the pericardial cavity, and indeed determining its shape although by no 

 means filling it, projects the myoepicardial mantle. Its ventral and lateral surfaces 

 are free; above and below it is reflected back as the parietal mesothelium, while dor- 

 sally this same reflection constitutes the posterior mesocardium. The two layers 

 of the mesocardium are, however, widely separated, for between them lies the 

 ventral keel of the fore-gut, even beyond what may be considered as the ventral 

 limits of the mesocardium, while farther cephalad is the bulging floor of the pharynx. 

 In the region of the dorsal recesses the two layers of mesocardium are most closely 

 approximated but still separated by the fore-gut. It is this undermining which 

 will soon free the heart except at its two ends, the communication thus established 

 between the two sides persisting as the transverse sinus of the pericardium. 



The myoepicardial mantle or, to be brief but less exact, the heart, is prac- 

 tically straight and of almost uniform width, but slightly broader below; its ventral, 

 tree surface is distinctly convex antero-posteriorly, as well as from side to side. 

 This ventral surface is rather more strongly convex below than above, and again 

 along its left margin, and passes insensibly into the lateral surfaces. Of these 

 the left is the simpler, presenting caudally and dorsally a slight bulging due to the 

 undermining just dorsal to it, The right side is throughout more strongly convex 

 and is traversed about its middle by a deep transverse furrow below which the 

 heart is distinctly thinner dorso-ventrally. The ventral end of this furrow is con- 

 tinuous with a much shallower, broader groove which passes caudad and slightly 

 mesad over the ventral surface of the heart, The deep furrow on the right, together 

 with a much more diffuse but roughly corresponding depression on the left, brings 

 about a slight constriction of the heart about or a little below the middle. This 

 constriction corresponds approximately to the anterior part of the narrowest por- 

 tion of the cardiac plexus. The relief of the right side of the heart is in marked con- 

 trast with that on the left, in that the former shows a well-marked bulging above the 

 transverse groove, while below this groove is a small, but very distinct, shoulder- 

 like prominence. On the left side is a slight prominence anteriorly which is sepa- 

 rated by a gentle concavity from a more pronounced elevation at the caudal end of 

 the heart. It must be remembered that what we have just described as the heart- 

 namely, the myoepicardial mantle— particularly the very anterior end of it, has its 

 form determined far more by the pharynx beneath it than by the vascular primordia 

 wit Inn. To what extent the form of the mantle and its external relief are influenced 

 at this stage by the structures beneath is a question, but it would seem that the 

 development of the mantle is in advance of the endothelial elements within, and so, 

 in a measure, independent of them. Further, there are found here not only the 

 future heart, but also the ventral aorta and the beginning of the first arches. 



In a structure relatively so short and really comprising at one end less and at 

 the other end more than the future heart, one can not expect to find much indica- 

 tion of the later differentiation of the tubular heart, The moderate constriction in 

 the mantle described above really does not involve the ventral surface of the heart, 

 If we identify the deep incisure on the right side as the beginning bulbo-ventricular 

 groove, then! but only as regards the mantle, the atrial and descending portions of 



