WITH SPECIAL REFERENCE TO THE VASCULAR SYSTEM. 



81 



effected no connection with the ccelom elsewhere, although we can not deny the 

 possibility of such a connection on the left side. From the deepest point of the 

 recess on the left side there extends backward for a short distance into the paraxial 

 mesoderm (that is, into a region definitely lateral to that in which more caudally 

 the myoccels are to be found) a very slender cleft which possibly, at its posterior 

 end, is in relation with some of the ccelomic spaces already noted (p. 56) as ingrowths, 

 in part at least, from the extra-embryonic ccelom. A second less notable cleft appears 

 at a level slightly more caudad. On the right side essentially the same picture is 

 presented as described above, except that the smaller, more caudal extension is 

 missing. As may be seen, the dorsal recesses are much nearer the floor than the 

 roof of the pericardial cavity; later this relation is reversed. However, it may be 

 noted in passing that the upper part of the pericardial cavity is in relation with 

 vessels which will later come to lie without this cavity, viz., the aortic arches. 

 Under the inner, mesial wall of these recesses lie the dorso-lateral grooves of the fore- 

 gut as previously noted; the future pulmonary anlage is more ventral and possibly 

 also farther cephalad. Anteriorly they are roofed in by the somatopleure near its 

 reflection to form the amnion. Their floor below is splanchnopleure, which mesially 

 and ventrally forms a sort of sling, constituting the floor of the pericardial cavity, the 

 whole being the septum transversum, around the dorsal border of which, at the 

 anterior intestinal portal, the fore-gut and mid-gut communicate. 





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Figure A. — Section 12-3-6, X90. The line of this section is shown in figure 3 on plate 2. Transversely the plane of 

 section is somewhat oblique, so that structures on the left of the median line (above in the illustration) are 

 cut at a higher level than those on the right. The section cuts just behind the primitive node, through the anterior 

 intestinal portal and approximately, though obliquely, through the middle of the heart. On the right in the figure 

 appears the body-stalk with the two umbilical veins and allantois between them. Three cavities are seen: 

 above in the illustration is the amniotic cavity; below is the yolk-sac, communicating anteriorly with the foregut; 

 farther forward, in the median line, is the pericardial ccelom with its contained heart. The central, densely cellular 

 mass is a tangential section of the left neural fold. Between the entoderm of the foregut and the superficial 

 ectoderm, the mesoderm can be traced forward to the point where it splits to become continuous with the visceral 

 and parietal layers of the pericardial cavity. (Cf. also text and fig. 11, plate 5.) 



