260 THE CHICK. 



The optic vesicles arise, early on the second day, as a pair of 

 lateral outgrowths from the fore-brain (Fig. Ill, BO). They give 

 rise, as in the frog, to the retina and the retinal pigment of the 

 eye, and their developmental changes will be described in the 

 section dealing with the formation of the eye (p. 275). 



The cerebral hemispheres. About the middle of the second 

 day, the fore-brain (Fig. Ill, BF) begins to grow forwards, in front 

 of the optic vesicles, as an anterior, median outgrowth, the vesicle 

 of the hemispheres. At the same time cranial flexure becomes 

 pronounced (Fig. 112), owing to the dorsal surface of the head 

 growing faster than the ventral surface ; the axis of the brain 

 becoming a curved instead of a straight line, and the fore-brain 

 being carried round to the ventral surface of the head. The 

 curvature of the brain progresses rapidly ; the fore-brain (Fig. 

 113) becoming placed at right angles to the rest of the brain, 

 and the mid-brain growing forwards so as to lie at the extreme 

 anterior end of the head. 



The vesicle of the hemispheres grows rapidly, both in length 

 and width : during the third day the paired cerebral hemispheres 

 arise from its anterior end as thin-walled outgrowths, separated 

 by a median furrow. The hemispheres (Figs. 113, 115, and 

 123, BS) enlarge rapidly, growing upwards and forwards, and 

 forming a pair of prominent rounded swellings at the anterior end 

 of the head, very conspicuous in embryos of the third to the seventh 

 or eighth day. From the ventral surface of their anterior ends 

 the olfactory nerves arise at a very early stage. 



From the eighth day onwards the hemispheres, though still 

 increasing in size, become less conspicuous from the surface, owing 

 to the forward growth of the face, and especially of the beak, which 

 elongates rapidly and completely alters the shape of the head 

 (Fig. 116). As the beak extends forwards, the anterior ends of the 

 hemispheres, from which the olfactory nerves arise, grow out as a 

 pair of small hollow buds, the olfactory lobes (Fig. 116, BY), from 

 the ends of which the olfactory nerves run forwards to the nose. 

 The walls of the hemispheres are at first thin ; in the later 

 stages they thicken considerably, especially on the outer side of 

 their hinder ends, where they form the corpora striata. The 

 cavities of the hemispheres persist throughout life as the lateral 

 ventricles of the brain, which retain their communication with 



