136 



THE HUMAN EMBRYO. 



OP, 



the deep bend in the segmented region of the body so as to constitute at the 

 dorsal outline of the embryo at that point a U-shaped curve (Fig. 71). This 

 bend is known as the dorsal flexure. Embryos of earlier stages have an in- 

 dication of this flexure, as shown in figure 69. Until we have intermediate 

 stages we cannot be sure that the assumption which seems natural is also cor- 

 rect ; namely, that the deep dorsal flexure of figure 7 1 is merely an accentua- 

 tion of the cavity on the dorsal side of the embryo 

 in earlier stages. In older embryos the dorsal flex- 

 ure is normally absent (compare Fig. 73 and the 

 following figures). It is possible that the change 

 from the concave to the convex position is very 

 abrupt, and it is not improbable that the time of the 

 occurrence of this change is variable. The head of 

 the embryo and the tail both project far beyond 

 the yolk-sac, which, however, still shows a broad 

 attachment to the embryo. The right-angled 

 head bend is well marked and the region of the 

 fore-brain projects downward so as to leave a de- 

 pressed area between the head and the heart. This 

 depression corresponds to the position of the oral 

 cavity. The heart is large, protuberant, and 

 considerably bent, so that we can distinguish its 

 three primary limbs. From the under side of the 

 caudal end of the embryo springs the stout body- 

 stalk by which the embryo is united with the villous 

 chorion. In another figure of that embryo there 

 were twenty-nine segments present. Above the 

 heart on the side of the pharyngeal region two ex- 

 ternal depressions are visible corresponding to the 

 first two gill clefts. They are elongated in a dorso- 

 ventral direction and are narrow. This position of 

 the amnion is well shown in figure 71. It arises 

 from the body-stalk at the side of the embryo along 



the yolk-sac and cardiac region, and extends around the embryo, but is not 

 yet fitted closely. 



The anatomy of this stage is known to us chiefly through the observations 

 of His upon two embryos designated by him as Lg and Sch. i . Lg measured 2.15 

 mm. ; Sch. i , 2.20 mm. The two embryos resemble one another closely. The fol- 

 lowing description applies especially to Lg. The anatomy can be understood from 



FIG. 72. RECONSTRUCTION OF THE 

 ANATOMY OF THE EMBRYO 

 SHOWN IN FIG. 71. 



Op, Optic vesicle, o.pl, Oral plate. 

 Ht, Endothelial heart. Li, 

 Liver. Om, Omphalo-mesaraic 

 vein. Yk, Yolk-sac. All, 

 Allantoic diverticulum formed 

 by the entoderm. u.v, Umbili- 

 cal vein. Ao, Aorta. Of, Oto- 

 cyst. (After W. His.) 



