WITH SPECIAL REFERENCE TO THE VASCULAR SYSTEM. 83 



the ventricular loop are conspicuously shorter than the remainder from which will 

 arise the ascending limb of the loop and the bulbus. This prominent shoulder on the 

 right is very similar to that shown in cat embryos (Schulte, 1906), except that here, 

 due to the late fusion of the mantles, the same prominence with the bulbo-ventricu- 

 lar groove above is formed on both sides, in each half of the myoepicardial mantle. 

 In the heart of Xo. 391 (2 mm.) as shown by Mall (1912), the groove is farther from 

 the attached, venous end of the heart and is, moreover, in its proper position on the 

 left side. Were the relations of the early endothelial tube within more in accord 

 with the form of the mantle one would feel less compunction in hanging on this little 

 projecture on the right side of the heart speculations a propos of an incipient dex- 

 trocardia or situs inversus, without, however, being able to offer much more to sub- 

 stantiate such a claim, or anything by way of explanation. 



Careful examination of the myocardial wall fails to reveal anything which 

 might be construed as distortion or post-mortem change of any kind. In the region 

 of the groove there is a slight thickening of the mantle and its free, ccelomic surface 

 is distinctly more irregular than elsewhere from the heaping up of its cells. At the 

 dorsal end of the bulbo-ventricular groove is a deep triangular depression continued 

 forward and backward along the line of the mesocardium. On the left side the 

 undermining of the mantle is well marked only in its caudal half, the deepest point 

 being distinctly below, caudad to the center of the mantle. The establishment of a 

 connection here across the median line, the future transverse sinus of the peri- 

 cardium, awaits only the withdrawal of the ventral keel of the fore-gut. This push- 

 ing inward of the mesocardium at a definitely more caudal point on the left side 

 (on the right this process is more extensive but also more anterior), together with 

 the deep groove on the right side and the fact that the ventral, transverse con- 

 vexity of the mantle is sharpest toward the left side of this surface, leads one to the 

 conclusion that there is developing in this case a reversed ventricular loop — that is, 

 a loop directed toward the left instead of toward the right. Another bit of evidence 

 which might be adduced in this connection is the fact that the right lateral border 

 of the cardiac plexus is more concave than the left, while the median fused portion 

 of the plexus shows a very definite keel, directed ventrally and to the left. This 

 last-mentioned character is most noticeable on section, as if the endothelial struc- 

 tures were reaching out toward the left, anticipating the formation of a ventricular 

 loop whose apex will be directed toward that side. 



As regards the relations of the myoepicardial mantle and its inclosed plexus, 

 the evidence here also points to a beginning reversed ventricular loop. The shallow 

 sagittal groove already noted on the ventral surface of the mantle in its caudal 

 third is situated slightly to the right of the open space between the omphalomesen- 

 teric roots; in other words, it overlies the left border of the expanded portion of the 

 right venous root. This sagittal groove is located to the right of the median line. 

 Traced upward, it turns outward, toward the right, to become continuous with 

 the ventral end of the bulbo-ventricular groove. Tin's shallow, sagittal furrow will 

 probably later be completely effaced, since we can hardly see that it might represent 

 the atrio-ventricular constriction. Whether or not it is an indication of a possi- 



