90 A HUMAN EMBRYO AT THE BEGINNING OF SEGMENTATION. 



DESCRIPTIONS OF PLATES. 



Plate 1. 



Fig. 1. Left lateral view of model of embryonic body viewed in the plane of section. X 100. Yolk-sac, amnion, 

 and body-stalk have been cut away. 



Fig. 2. Dorsal view of the same model. Both views show open neural groove. Otic plates and V ganglia appear 

 as darker, rounded areas, the former larger and farther lateral, in the region of the rhombencephalon. 

 Outlines of the underlying somites and in figure 2 the left umbilical vein. At the posterior end of the body 

 the broad, dark line indicates the primitive-streak region with the primitive node at its anterior end and 

 the cloacal membrane at the posterior end. Projecting from the cut surface of the body-stalk are the two 

 umbilical arteries, with their branches to form the ventral venous plexus (cf . figs. 3 and 4) ; between the 

 arteries is the allantois. In figure 1, first ectodermic pocket in front of left otic plate, opposite midbrain 

 flexure. 



Plate 2. 



Fig. 3. Chorion, body-stalk, and embryo, the last on sagittal section, view as in figure 1. X 90. Ectoderm is 

 shown in yellow, entoderm in green, vessels in red. Chorion shows numerous small vessels and the slender 

 connection with the large venous plexus of body-stalk. Lower anastomosis between umbilical artery and 

 ventral venous plexus not shown. Distal portion of allantois in contact with mesothelium of body-stalk. 

 Primitive streak between primitive node and cloacal membrane. Heart plexus suspended between 

 myoepicardial mantle and fore-gut (cf. figs. 7 and 8 on plate 4). Line of section 12-3-6, text-figure a, 

 is indicated. 



Plate 3. 



Fig. 4. Vascular system as seen from the right side. X 100. Color scheme as in figure 3. The ectoderm has been 

 cut away close to the border of the medullary folds. The mesoderm of the embryo and adjacent yolk- 

 sac has been removed to show the vessels embedded in it. In the vitelline plexus and that part of it which 

 represents the yolk-sac portion of the vitelline (umbilical) artery, the endothelial vessels are represented 

 in darker color than the less definitely lined channels. The same is true for the anterior portion of the 

 dorsal aorta and the isolated vesicles in the roof of the fore-gut. Scattered blood-islands and other vascu- 

 lar formations are seen through the mesoderm of the yolk-sac. Anterior to the branch of the aorta (marked 

 vitelline artery) is a second, more doubtful connection between the aorta and the vitelline plexus. 



Plate 4. 



Right ventrolateral view of heart, myoepicardial mantle, and pericardial cavity. X 100. Ventral wall, 



toward exocecelom, has been removed (cf. fig. 3). 

 Ventral view of same model, slightly from the left side. The vein in the splanchnopleure on the right side 



is the yolk-sac portion of the omphalomesenteric vein, as yet unconnected with the heart. 

 Right ventrolateral view of cardiac plexus. X 200. 

 Same model in ventral view. For position of heart plexus within the body, see figures 3 and 4. 



Plate 5. 

 (Further details concerning the figures on this plate will be found in the text.) 



Fig. 9. Section 12-1-2. X 350. Portion of body-stalk low down, showing blood-islands in left umbilical artery. 

 Toward the yolk-sac, above in the illustration, the lumen of the artery communicates freely with the 

 spaces in the mesenchyme. 



Fig. 10. Section 12-1-6. X 350. Right umbilical artery in body-stalk, at a slightly higher level than figure 9. The 

 vessel wall is here much better defined than that shown in figure 9. The blood-island i£ also less dense 

 and almost free within the vessel. Toward the yolk-sac the changes in the vessel wall and relation of its 

 lumen to the wide tissue spaces in front can be seen. This represents the first indication of a connection 

 between the vessels in the embryo and those of the body-stalk; in this case between the right dorsal aorta 

 and the right umbilical artery, through the intermediation of the caudal portion of the vitelline plexus 

 and of the vitelline artery. 



Fig. 11. Section 12-3-5. X 350. Detail of text-figure a. Section through heart, about the middle of the organ. 

 The thickening of the mantle on the right side of the heart is only apparent, being due to the sudden 

 change in curvature at this level. 



Fig. 12. Section 12-2-6. X 600. Caudal end of left dorsal aorta, represented by an elongated blood island. Begin- 

 ning lumen formation near its posterior end. The mesoderm in this region has not yet begun to segment. 



