THE OPTIC VESICLES. 



95 



strictions. which give rise to the stalks take place chiefly from 

 above downwards, and also somewhat inwards and backwards. 

 Thus from the first the vesicles appear to spring from the 

 under part of the fore-brain. 



FIG. 26. 



cc 



jiOA 



SECTION THROUGH THE HIND-BRAIN OF A CHICK AT THE END OF THE THIRD 

 DAY OF INCUBATION. 



IV Fourth ventricle. The section shews the very thin roof and thicker sides 



of the ventricle. 



Ch. Notochord (diagrammatic shading). 



CV. Anterior cardinal vein. 



CC. Involuted auditory vesicle. CC points to the end which will form the 

 cochlear canal. RL. Kecessus labyrinth!, liy. hypoblast lining the alimen- 

 tary canal, hy is itself placed in the cavity of the alimentary canal, in that 

 part of the canal which will become the throat. The lower (anterior) wall of 

 the canal is not shewn in the section, but on each side are seen portions of a pair 

 of visceral arches. In each arch is seen the section of the aortic arch A OA 

 belonging to the visceral arch. The vessel thus cut through is running upwards 

 towards the head, being about to join the dorsal aorta AO. Had the section 

 been nearer the head, and carried through the plane at Which the aortic arch 

 curves round the alimentary canal to reach the mesoblast above it, A OA and A 

 would have formed one continuous curved space. In sections lower down in 

 the back the two aorta, A 0, one on either side would be found fused into one 

 median canal. 



The shading of the mesoblast is diagrammatic ; it is here a uniform mass 

 of spindle-shaped cells ; there being in this region no differentiation into proto- 

 vertebre. 



