EMBRYO OF SEVENTEEN SEGMENTS 47 



spinal cord. It shows a number of secondary constrictions, the neurom- 

 eres. The ectoderm is thickened laterally over the optic vesicles to form 

 the lens placode of the eye (Fig. 43). The optic vesicle is flattened at this 

 point and will soon invaginate to produce the inner, nervous layer of the 

 retina. Dorso-laterally, in the hind-brain region, the ectoderm is thick- 

 ened and invaginated as the auditory placode (Fig. 45). This placode 

 later forms the otocyst, or otic vesicle, from which is differentiated the 

 epithelium of the internal ear (membranous labyrinth). 



Digestive Tube. 'The entoderm is still flattened out over the surface 

 of the yolk caudal to the intestinal portal. In Fig. 41 the greater part of 

 the entoderm is cut away. The flattened fore-gut, folded inward at the 

 portal, shows indications of three lateral diverticula, the pharyngeal 

 pouches. Cephalad, the pharynx is closed ventrally by the pharyngeal 

 membrane. 



Hind-brain Fore-gut Neural tube 



Mid-brain. 



Fore-brain 

 Amnionfold. 



^.ntestinal portal 



~^.,, , Vitelline vein 



Pharyngeal membrane v ', 



aorta Dorsal Heart Pericardial cavity 

 mesocardium 



FIG. 42. A median longitudinal section of the head of a thirty-eight-hour chick embryo. 



X about 50. 



Heart and Blood Vessels. After receiving the vitelline veins cepha- 

 lad of the intestinal portal, the double-walled tube of the heart dilates 

 and bends ventrad and to the embryo's right (Fig. 41). It then is flexed 

 dorsad and to the median plane, and narrows to form the ventral aorta. 

 The aorta lies ventrad to the pharynx and divides at the boundary line 

 between the mid- and hind-brain into two ventral aortas. These diverge 

 and course dorsad around the pharynx. Before reaching the optic vesicles 

 they bend sharply dorsad and caudad, and, as the paired descending aortae, 

 may be traced to a point opposite the last primitive segments. In the 

 region of the intestinal portal they lie close together and have fused to 

 form a single vessel, the dorsal aorta. They soon separate, and, opposite 

 the last primitive segments, they are connected by numerous capillaries 

 with the vascular network. In this region, at a later stage, the trunks 

 of the paired vitelline arteries will be differentiated. The heart beats at 

 this st? 6 e; the blood flows from the vascular area by way of the vitelline 

 veins UD the heart, thence by the aortae and vitelline arteries back again. 



