320 IVAN E. WALLIN 



caudal direction and its lateral width is greatest at the cephalic 

 end. Caudal and to the right of the yolk stalk the belly stalk 

 leaves the embryo passing ventrally and curving to the right 

 and caudad. Lateral to the yolk stalk the embryonic coelom 

 has an extensive communication with the extra-embryonic coelom. 



The heart produces a prominent bulging of the right side of 

 the bod}^ immediately caudad to the head. The most prominent 

 part of the bulging marks the flexure in the heart tube between 

 the bulbus cordis and the ventricle. The neck flexure has not 

 advanced to any prominent degree. There are two prominences 

 on the dorsal surface of the head region, one at the cephalic end 

 of the mid-brain and the other at the cephalic end of the hind- 

 brain. Caudally the bod}^ curves gradually in a ventral direc- 

 tion. There is no distinct caudal flexure. 



The medullary tube is open to the exterior at both ends. The 

 cephalic neuropore exhibits an unusual appearance for an embryo 

 of this age. It is very wide and gives a great breadth to the 

 head when viewed from the ventral aspect. The lateral lips of 

 this neuropore curve dorsally and form the ventral boundary of 

 a deep groove which is directed cephalo-caudally. The caudal 

 end of this groove nuis into the stomodeum. This part of the 

 nervous system which represents the forebrain has not kept 

 apace with the development of the remainder of the tube. It 

 apparently is a persistence of the condition which is present in 

 an earlier stage of development. Eternod's ('95) embryo of 

 eight somites and the embryo of seven somites described by 

 Dandy ('10) exhibit cephalic neuropores which appear to be in 

 about the same stage of development. 



There are no indications of otic invaginations. Two pairs of 

 entodermal pouches are in contact with the ectoderm. The 

 points of contact are indicated on the surface by shallow depres- 

 sions. In figure 2 their positions have been indicated on the sur- 

 face by broken lines. 



The amnion lies close on to the body of the embryo. The 

 head fold crosses the ventral aspect of the heart at about its 

 middle. The lateral folds follow the lateral lips of the coelom. 

 The tail fold is situated on the dorsal aspect of the belly stalk. 



