80 A HUMAN EMBRYO AT THE BEGINNING OF SEGMENTATION, 



very definitely lined, thin-walled vessel — the left umbilical vein (figs. 2 and 6). 

 It is of considerable antero-posterior extent and in its cephalic portion is a very 

 sharply defined channel, but with extremely thin walls. At this end it runs out as 

 a solid spindle-shaped sprout in a region almost devoid of cells. Its posterior extrem- 

 ity is gradually lost in the loose mesenchyme under the ectoderm. The vein pre- 

 sents two or three mesial branches, more distinct in front, through which a connec- 

 tion will be set up with the earlier formed omphalomesenteric root of the heart, the 

 vitello-umbilical vein. Conditions on the right side are too uncertain to warrant 

 description. On both sides, however, there occur, farther back, scattered vaso- 

 factive cells and small angiocysts in the somatopleure, or even in the amnion close 

 by, marking out the course of the umbilical vein backward toward the body-stalk. 

 At present, however, the embryonic part of the umbilical vein is entirely isolated 

 and independent. A few short branches come off the venous plexus of the body- 

 stalk at its lower ends; these are more conspicuous on the right side and may aid 

 later in completing the afferent umbilical channel. 



MYOEPICARDIAL MANTLE AND PERICARDIAL CCELOM. 



The pericardial cavity and myoepicardial mantle, the latter with its contained 

 plexiform heart already described, are of particular interest at this stage (text-figure 

 A, and figs. 5, 6, and 11). In transverse section the cavity throughout is crescenticin 

 outline and corresponds, in its antero-posterior extent, quite closely with that of 

 the fore-gut. The floor of the cavity is concave, narrow, and roughened internally 

 by a number of small, radiating ridges in the thickened mesothelium. The cephalic 

 limits of the cavity project upward as two narrow slits on either side of the fore-gut 

 (or mouth) lateral to the buccopharyngeal membrane. Similar diverticula of the 

 pericardial cavity have been noted in a 4.9 mm. embryo (Ingalls, 1907, 1. c, p. 553). 

 The recess on the right side is somewhat more extensive, terminating in three finger- 

 like prolongations. As the dorsal wall of the cavity we may describe the region 

 about the two extremities or horns of the crescent which the cavity presents on 

 section. Cephalad this is nothing more than the bottom of a narrow cleft between 

 the splanchnic mesoderm (myoepicardial mantle) and the somatopleure. Farther 

 back this cleft gradually widens out, especially on the right side, due in part to the 

 narrowing of the mesocardium — a sort of undermining. 



This increase in width continues to a point about opposite the slight con- 

 striction which can be seen in the myoepicardial mantle. Just below, i. e., posterior to 

 this, there is a sudden increase in the transverse dimensions of the cavity, due to 

 the presence at this level of two deep, well-marked, dorsally or dorso-caudally 

 directed, roughly funnel-shaped diverticula which penetrate for some distance into 

 the mass of mesoderm which farther back is beginning to break up into segments. 

 These diverticula take origin from the widest part of the pericardial ccelom and more- 

 over close to its lateral wall on either side. Mesially they are in relation with, or 

 even extend dorsally beyond, the dorso-lateral grooves of the fore-gut. Of the 

 outpocketings just noted (recessus parietales dorsales) the right is better defined 

 and more extensive in all its measurements, and is possibly situated a trifle 

 more cephalad than the left. As far as can be determined they are yet blind, having 



