EMBRYO OF TWENTY-SEVEN SEGMENTS 



55 



CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS (FIFTY HOURS) 



This embryo, of nearly fifty hours' incubation, lies in the center of the vascular 

 area and is pecuUar in that the head is twisted 90° to the right. In a dorsal view, 

 therefore, one sees the right side of the head but the dorsal side of the body. In 

 the region of the mid-brain is a very marked bend, the cephalic flexure. Below 

 the head, and ventral in position, Kes the tubular heart, now bent in the form of 





id-brain 



1 re-brain 

 - Opiic vesicle 



i 



~i Li lis vesicle 



) 



t ( utricle of heart 



-^ I ileUine vein 



1 



R vitelline artery 



Neural 



HI ' 4 



-^.^.±PAAre 



Primitive streal L*Si' •' 



and tail bud Z ~^ '^ 



Fig. 56. — Dorsal view of a fifty-hour chick embryo, stained and mounted in balsam. X 14. 



a letter S- Dorsal to the heart, in the region of the pharynx, three transverse 

 grooves or sKts may be seen. These are the branchial clefts or gill slits. The 

 head of the embryo is now covered by a double fold of the somatopleure, the 

 head fold of the amnion. It envelops the head like a veil. Caudally, a fold and 

 opacity mark the position of the tail bud from which develops the caudal end of 

 the body. The curved fold embracing this is the tail fold of the amnion, which will 

 eventually meet the head fold and completely envelop the embryo. 



Central Nervous System and Sense Organs (Fig. 57). — Cephalad, the neural 



