22 LABORATORY OUTLINES FOR EMBRYOLOGY 



the mid-brain form a right angle with the rest of the head. 

 This bend is called the cephalic flexure. The head is now 

 completely separated from the blastoderm, and the body, 

 which has been continuous with it on the sides, is partly 

 separated from it by the lateral folds, which unite with the 

 head fold. The body has become twisted a short dis- 

 tance back of the region of the heart, so that the head is 

 now lying on its side. Which side? Gan you see both 

 optic vesicles? Why? Compare the size of the optic 

 vesicles with that of the fore-brain. What were their 

 relative sizes in the thirty-six-hour chick? The outer 

 part of the optic vesicle has invaginated into the original 

 cavity and has thus formed a cup, the optic cup. The 

 walls of this double cup form the two layers of the retina, 

 which may be seen at this time. What is the compara- 

 tive thickness of these two layers? The place where the 

 rim of the cup does not appear to be continuous is the 

 choroid fissure. The outside ectoderm in the region of 

 the optic cup has thickened and formed an invagination 

 into the optic cup. This is the lens. It appears as a 

 sphere lying in the optic cup. In the region of the hind- 

 brain is an invagination from the outside ectoderm. This 

 is the auditory vesicle. 



The heart has elongated more rapidly than the body, 

 consequently it has become twisted upon itself. The 

 vitelline veins are in the lateral folds of the blastoderm. 

 About half way between the vitelline veins and the end of 

 the tail are two blood vessels extending from the embryo 

 into the blastoderm. These are vitelline arteries. They 

 carry the blood from the dorsal aortae to the blastoderm. 



