A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 120 



The head of the embryo is large and rounded. When viewed from in front 

 (plate 1, fig. 2), it appears somewhat egg-shaped. Toward its middle, on either side, 

 are distinct outward bulgings, beneath which lie the optic vesicles. The mouth is a 

 well-defined opening, limited on each side and below by the large mandibular arches. 

 Behind each mandibular arch there are three distinct gill-clefts. The first 

 two of these are long and narrow and are directed obliquely to the longitudinal 

 axis of the body, but the second bends dorsal ward in its upper portion at almost 

 right angles; the third is more shallow and rounded. The gill-clefts are bounded 

 by four distinct arches. The first of these, the mandibular, is large and rounded, 

 the second is similarly shaped but smaller, the third and fourth appear merely as 

 rounded eminences. 



The pericardium and heart lie just ventral to the arches. They form a large 

 bulging which is more prominent on the right than on the left. 



Beginning some little distance caudal to the last arch and placed at regular 

 intervals throughout the remainder of the embryo, the mesodermic somites can be 

 seen bulging through the skin. About 15 of these are perceptible from the surface, 

 10 in front and 5 behind the sharp flexure of the back. 



The amnion is reflected from the embryo at the lower end of the pericardial 

 cavity. The line of reflection here is curving and follows the lower curvature of the 

 pericardial wall. At the side of the embryo, about halfway between the dorsal 

 and ventral mid-lines, the line of reflection turns caudally, passes along the sides of 

 the yolk-stalk, along the sides of the embryo, and finally on the body-stalk. Thus 

 all the head, pericardial wall, and most of the caudal extremity and the dorsal portion 

 of the remainder of the body of the embryo are within the amnionic sac. 



Ventral to the line of reflection of the amnion is the yolk-sac. This is a large, 

 irregular vesicle, broken through in places and flattened laterally. The yolk- 

 stalk, which proceeds from the embryo just beneath the pericardial sac, is also 

 flattened, but from above downward. 



The outer surface of the hind gut, /. e., its mesothelial surface, is distinctly 

 seen turning backward to follow the curvature of the caudal extremity. At its 

 bend there are two projections of the body-cavity, which likewise pass backward 

 into the tail. 



The body-stalk is attached to the embryo near its lower end, ventral to the 

 caudal extremity. It turns sharply backward. The amnion is reflected from its 

 upper surface for a short distance beyond the embryo. As a whole the body-stalk 

 is short and broad, gradually becoming broader as the chorion is neared. 



No indications of either fore or hind limb-buds can be found on the body of 

 the embryo. 



In comparing this embryo with Janosik's, it is seen that the back of the latter 

 presents a more even curvature, which extends to the tail of the embryo. The 

 angle formed at the top of the head (the midbrain bend) is almost identical in 

 both specimens. The head of Janosik's embryo, however, is pointed, while in my 

 specimen it is more rounded. In marked contrast to both of these specimens is 

 the Robert Meyer embryo No. 300, as modeled by Thompson. Here the head 



