SOCIETY OF THE UNIVERSITY Of ABERDEEN. 99 



more nearly approaches that of the embryo of eight paired somites and 

 a length of 211 mm. described and modelled by Professor Eternod (4). 

 Like the Eternod embryo, it is spirally twisted in its long axis, and 

 shows the same three back curvatures a convex neck bend, a con- 

 cave dorsal, and a convex rump. The dorsal flexure is slight, while 

 the neck bend makes a right angle with the body, approaching in this 

 respect the form of the embryo of twenty-three somites and a length 

 of 215 mm. described by Professor Peter Thompson (9). The medul- 

 lary plate is still open at its head and caudal ends. The front of the 

 head is directed down and slightly to the right over the bulging heart 

 region, and thus is formed a well-developed stomodaium. Three 

 visceral clefts are visible the first and second being deep, the third a 

 shallow groove. The first and second visceral clefts lie dorsal to the 

 rounded swelling caused by the commencing mandibular arch. The 

 first cleft occupies a position more to the cranial end of the embryo. 

 The second cleft is separated by a convex ridge from the first, and its 

 caudal end extends to the convexity of the body wall caused by the 

 heart. The third visceral cleft is caudal and somewhat more dorsal, 

 and separated by a considerable interval from the other two clefts. 

 Its ventral end leads into a depression of the body wall behind the 

 pericardial region. The ear pit is represented by an oval depressed 

 area of thickened epithelium just cranial to the dorsal end of the third 

 visceral cleft. There is a large elongated umbilical opening through 

 which the embryonic and extra-embryonic ccelom are in communica- 

 tion. The yolk sac is constricted at its communication with the primi- 

 tive intestine to form a distinct neck. The body stalk arises from the 

 ventral aspect of the embryo and passes to the right, lying first in front 

 and then to the right of the caudal end of the embryo, the tail being 

 directed to the left. The body stalk is separated from the neck of the 

 yolk sac by a narrow interval. The caudal end of the embryo is swollen 

 out, and terminates in a blunt point ; a trace of the primitive streak is 

 prolonged from the medullary groove to the cloacal membrane. Along 

 the line of attachment of the amnion to the lateral body wall on each 

 side there is a ridge caused by the underlying umbilical vein ; this ridge 



