44 POULTHY LABORATORY GUIDE 



Note the general shape of the embryo and any 

 change in the general position from day to day. 

 Examine the development of the brain as char- 

 acterized by the projections on the head. Lo- 

 cate the following: cerebral hemisphere, fore- 

 brain, mid-brain, and cerebellum. Examine 

 carefully the development of these, and note the 

 rapid development of the optic vesicle and the 

 growth of the lens. Just above and a Uttle to 

 the rear of the optic vesicle will be seen the 

 auditory pit or vesicle. At the base of the head 

 fold, below the optic vesicle, will be seen the 

 heart, and running from it are the nmnerous 

 veins and arteries which form the vascular 

 system. The heart begins to beat and blood to 

 flow during the second day. The vascular sys- 

 tem increases in size very rapidly, and at the end 

 of the third day is nearly complete. 



Passing down the body of the embryo in the 

 centre will be seen a dark line, which is called 

 the noto-cord, and develops into the spinal cord. 

 On each side of this cord will be seen small dark 

 projections called the proto-vertebrse, which later 

 form the bones of the spinal column. On the 

 third day the embryo turns on its left side, the 

 head bends downward, the vitelline circulation is 

 completed, and the events after that day are 

 distinguished by the extension of parts already 



