CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS 65 



For the histological development (histogenesis) of the various tissues from 

 the primary germ layers see Chapter X. 



CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS (FIFTY HOURS) 



This embryo, which is taken as a type of the forty-eight to fifty- two-hour 



stage, lies in the center of the vascular area and is peculiar in that the head is 



twisted 90 to the right. We therefore see the right side of the head but the dorsal 



side of the body. In the region of the mid-brain is the very marked head-bend 



Hind-brain, 

 Ofic vesicle 

 Bronchi at clefts 

 Ammon fold. 



/Itrium 



Lvlfellint artsy 

 Neural tube 



R-imitii/e sfreaff 



Mid-brain 



Fort-brain 

 Optic veskfe 

 Lens vesicle 



Venlric/e of heart 

 Vifel/i'ne vein 



ft. vitelline arlery 



Segment 4- 

 Area pelludda. 



FIG. 54. Dorsal view of a fifty-hour chick embryo, stained and mounted in balsam. X 14. 



or cephalic flexure. Below the head and ventral in position lies the tubular heart, 

 now bent in the form of a letter S. Dorsal to the heart in the region of the 

 pharynx, three transverse grooves or slits 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. 

 S 



