256 THE CHICK. 



borne in mind that this is the- condition in which it remains 

 throughout life in the frog, and in many fish. 



The mid-brain undergoes comparatively slight changes. Up 

 to the end of the fourth day it is approximately spherical in 

 shape ; and, owing to its great size and the position which, 

 through cranial flexure, it occupies at the apex of the head, it 

 plays a prominent part in determining the shape of the embryo 

 (Figs. 113 and 115, BM). 



On the fifth day, the optic lobes begin to grow out as a pair 

 of rounded swellings from the roof of the mid-bi-ain, separated by 

 a median longitudinal groove. These steadily increase in size 

 during the following days ; up to the sixteenth day they remain 

 in close contact with each other, but during the last few days of 

 incubation they become pushed apart by the forward growth of 

 the cerebellum, and take up the position at the sides of the brain 

 characteristic of the optic lobes in the adult bird. 



The floor of the mid-brain, and the sides, ventral to the 

 optic lobes, become greatly thickened by the formation of the 

 crura cerebri. The cavity of the mid-brain becomes greatly 

 reduced by this thickening of its floor and sides, and forms the 

 Sylvian aqueduct of the adult. 



The thalamencephalon is formed from the original anterior 

 cerebral vesicle, or fore-brain (Fig. Ill, BF). 



The roof and floor of the thalamencephalon remain thin 

 throughout life, but the sides thicken very greatly to form the 

 optic thalami, reducing the central cavity to a narrow vertical 

 cleft, the third ventricle of the adult (cf. Fig. 116, BF). 



The anterior wall of the thalamencephalon forms a thin and 

 narrow band, the lamina terminalis (Fig. 116, BT), which lies 

 between the roots of the two cerebral hemispheres : in con- 

 nection with this, the anterior commissure is developed as a 

 narrow transverse band of nerve fibres, running across between 

 the basal parts of the hemispheres. 



The roof of the thalamencephalon, like that of the fourth 

 ventricle, becomes early reduced, along the greater part of its 

 length, to a single layer of epithelial cells, devoid of nervous 

 elements. About the middle of its length, the pineal body arises, 

 at the commencement of the third day, as a hollow, rounded, 



