THE 48-HOUR EMBRYO 219 



infundibulum forms the posterior lobe. The section shows two divisions of 

 the brain — the diencephalon and the myelencephalon. 



In the region of the pharynx the first visceral pouches extend laterally 

 and meet the first visceral groove. A section of the mesenchyme is cut off 

 to form a visceral arch, and the artery running through this mesenchyme is 

 the first aortic arch. Just above the pharynx the two large vessels with some 

 blood cells in them are the dorsal aortae. On either side of the hindbrain are 

 the anterior cardinal veins. 



The lens of the eye is a derivative of the head epidermis. Shortly after 

 the optic vesicles make contact with the epidermis, a thickening appears. This 

 invaginates (Fig. 137) and finally separates from the surface layer of 

 epidermis. The resulting hollow body is the lens vesicle. 



The section shows many other interesting features. The floor of the 

 diencephalon evaginates and touches the epidermis, which is somewhat 

 invaginated. This is where Rathke's pouch opens into the stomodeum. From 

 the walls of the pharynx the first and second visceral pouches have grown 

 out to make contact with the ectoderm and two pairs of aortic arches and 

 visceral arches are formed. One of the aortic arches is very close to a dorsal 

 aorta, and in an adjacent section the two join. Anterior cardinal veins are 

 present. The edge of the first somite shows one side of the hindbrain. 



A section of the 48-hour embryo taken just posterior to the lens reveals 

 another epidermal structure, the otocyst (Fig. 138). Its development is very 

 similar to that of the lens. Lateral to the myelencephalon two thickenings of 

 the epidermis appear. They fold to form invaginating vesicles which later 

 separate from the adjacent epidermis. The vesicles are called otocysts, or otic 

 vesicles. One of the otic vesicles is touching the epidermis, and a few 

 sections anterior to that shown in Figure 138 the cavity of the otocyst is open 

 to the exterior. The section also includes part of the optic cup and shows the 



DIENCEPHALON 



MYELENCEPHALON 



Fig. 1 38. Otocyst and optic stalk of a 48-hour chick embryo. 



