EMBRYO OF SEVEN SEGMENTS 



49 



tubular heart lies ventral to the fore-gut and cranial to the fovea cardiaca. In 

 later stages it is bent to the right. Converging forward to the heart on each side 

 of the fovea are the vitelline veins just making their appearance at this stage. 

 The lips of the neural folds have met throughout the cranial two-thirds of the 

 embryo but have not fused. The neural tube formed thus by the closing of the 

 ectodermal folds is open at either end. Cephalad, the neural tube has begun to 



inferior neuropore rorebroin 



Pharynx 



i ' ""^fSBfe* y Frse P° rf ' on °f h e °d 



Fovea cardiaca 



Left vitelline 

 vein 



Mesodermal |} § 

 Segment 3 



WJcMLffignt vitelline 

 "''"'"8 vein 



"-WT 3 Neural groove 



|_ Segmental 

 zone 



$£. Primitive node 



f 



f- Blood island 



Primitive sfreaK 



Fig. 31. — Dorsal view of a twenty-five-hour chick embryo with seven primitive segments. X 20. 



expand to form the brain vesicles. Of these only the fore-brain is prominent, 

 and from it laterally the optic vesicles are budding out. The paraxial mesoderm 

 is divided by transverse furrows into seven pairs of primitive segments. Caudally 

 between the segments and the primitive streak there is undifferentiated meso- 

 derm, but new pairs of segments will develop in this region. Looking through the 

 open neural tube (rhomboidal sinus), one may see in the midline the chorda dor- 

 salis extending from the primitive node cephalad until it is lost beneath the 



