24 LABORATORY OUTLINES FOR EMBRYOLOGY 



passes through the body twice, the fore-brain and the 

 hind-brain. The ventral sides of the two parts He to- 

 gether. Of what germ layer is the brain formed? The 

 optic vesicle? The lens? The outer covering of the 

 body is ectoderm. The part of the body in which the 

 fore-brain is located is enclosed in two membranes, and 

 the part in which the hind-brain is located is almost, if 

 not entirely, enclosed in these two membranes. The 

 membrane next to the body is the amnion and is com- 

 posed of two layers of cells: an inner layer, the ectoderm, 

 and an outer layer, the mesoderm. On the side next to 

 the yolk is the yolk sac, composed of an inner layer of 

 mesodermal cells and an outer layer of endodermal cells. 

 It should be kept in mind that the mesoderm of the yolk 

 sac contains the blood vessels. On the side away from 

 the yolk is the chorion, composed of an inner layer of 

 mesodermal cells and an outer layer of ectodermal cells. 

 What is the shape of the optic vesicle? What is the 

 relative thickness of the layers of the optic vesicle? Is 

 the lens a solid mass of cells? Is it connected to the 

 outside ectoderm or has it become separated? 



The hind-brain may be distinguished from the fore- 

 brain and the mid-brain on account of its thin roof. 

 What is the shape of the pharynx? The places where 

 the outside ectoderm comes almost in contact with the 

 endoderm of the pharynx are the gill clefts or visceral 

 clefts. The thickened parts between the visceral clefts 

 are the gill arches or visceral arches. In each visceral 

 arch is an aortic arch which carries blood from the ventral 

 aorta to the dorsal aorta. Remember that the ventral 



