HUMAN EMBRYO IN THE EIGHTH STAGE. 



141 



Human Embryo in the Eighth Stage with Two Gill-clefts Showing Externally. 



Several embryos in this stage have been described and some of them studied 

 anatomically. Those which are best preserved and which we have best reason 

 to think are normal present a very singular appearance, owing to 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. 86). 



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

 indication of this flexure, as shown in figure 84. Until we have intermediate 

 stages we cannot be sure that the assumption which seems natural is also correct; 

 namely, that the deep dorsal flexure of figure 86 

 is merely an accentuation of the cavity on 

 the dorsal side of the embryo in earlier stages. 

 In older embryos the dorsal flexure is normally 

 absent (compare Fig. 88 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, how- 

 ever, still shows a broad attachment to the 

 embryo. The right-angled head-bend is well 

 marked and the region of the fore-brain pro- 

 jects 'downward so as to leave a depressed 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 ^nd of the embryo springs the stout 

 body-stalk by which the embryo is united with the villous chorion. In another 

 embryo of this stage there were twenty-nine segments present. Above the heart 

 on the side of the pharyngeal region two external depressions are visible corre- 

 sponding 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 86. 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 

 following description applies especially to Lg. The anatomy can be understood 

 from the accompanying figure 87. The medullary tube extends the entire length of 

 the embryo and is the principal component of the head. From the region of the 

 fore-brain has been formed an outgrowth to constitute the optic vesicle, Op. At 

 the side of the hind-brain and on the dorsal side of the pharynx is situated the 



FIG. 86. HUMAN EMBRYO, 2.15 MM. LONG. 

 (After W. His.) 



