84 A HUMAN EMBRYO AT THE BEGINNING OF SEGMENTATION, 



ble earlier and greater independence of the two halves of the mantle, which appears 

 doubtful, must remain for the present an open question. At the point where the 

 grooves become confluent they are situated to the right of the plexus, so that the 

 shoulder on the right is quite out of relation with the plexus, containing only the 

 fine fibrillar network to be described later. On the whole, then, the endothelial 

 component of the heart is shifted a little toward the left within the mantle, besides 

 showing a slight but definite bowing toward the same side. While obviously the 

 plexus must follow the bending of the tubular mantle, made possible by the early 

 breaking through of the mesocardium, there is nothing at present to prevent the 

 plexus from undergoing considerable changes in position within, and independent of, 

 the mantle. All things considered, one can not well avoid the conclusion that we 

 have before us about the earliest recognizable stage of dextrocardia, infrequent 

 enough in the adult, to which might have been added later a more or less complete 

 situs inversus viscerum. 



The apparently low, caudal location of the bulbo- ventricular groove is to be 

 explained by conditions under the upper part of the mantle. There is found here, 

 yet in relation with the pericardial cavity, the anterior, most obviously plexiform 

 segment of the so-called heart plexus, which represents, however, the ventral 

 aortae, first arches, and very possibly other aortic arches (Bremer, 1912), as well as 

 the distal extremity of the bulbus. This portion of the mantle is also much thinner 

 than that farther back, where it will form the wall of the ventricular loop, and the 

 protoplasmic network is distinctly less delicate and regular. 



As regards the future subdivisions of the endothelial portion of the heart at 

 this stage, little more than their obvious spatial sequence can be recognized. In its 

 caudal, expanded third is represented the sinus venosus, the wide transverse extent 

 of which is already an accomplished fact. There remains only the complete fusion of 

 its two lateral components, already inaugurated by two small connecting channels. 

 The anterior third of the endothelial heart is frankly a plexus, the distal portion of 

 which is made up of bilaterally arranged, open endothelium-lined vessels. From 

 this segment must be derived the termination of the bulbus, the ventral aortae, and 

 the anterior aortic arches. The remaining unpaired middle third of the heart 

 plexus contains, therefore, the bulk of the future organ, aside from the mantle. 

 Any definite limits can not be given, but in the formation of the ventricular loop 

 which will arise from its rapid growth and elongation, it seems probable that the 

 caudal third of the plexus will be encroached upon more and drawn into the ventricu- 

 lar loop, as atrium, to a greater extent than the anterior third will be incorporated 



into the bulbus. 



Between the myoepicardial mantle, and the endocardial primordium within, 

 there exists a wide space winch is more extensive and characteristic in the ventral 

 and lateral regions than in the region dorsal to the heart plexus (text-figure A and 

 fig. 11). This interval is bridged over by innumerable fine protoplasmic fibers 

 which have their origin, or attachment, on the mantle externally and the surface of 

 the plexus internally. They were described long ago by His, and more recently 

 by Mall (1912), and there can be no doubt but that they represent an essential and 

 perfectly normal constituent of the embryonic heart, It may be granted, of course, 



