176 DEVELOPMENT DURING THE THIRD DAY. 



a thickening of the walls and roof of the front portion, separates 

 the hind-brain into the cerebellum and the medulla oblongata 

 (fig. 1 1 2 Cb and HB\ While the walls of the cerebellar portion 

 of the hind-brain become very much thickened as well at the 

 roof as at the sides, the roof of the posterior portion or medulla 

 oblongata thins out into a mere membrane, forming a delicate 

 covering to the cavity of the vesicle (fig. 114 IV\ which here 

 becoming broad and shallow with greatly thickened floor and 

 sides, is known as the fourth ventricle, subsequently overhung 

 by the largely-developed posterior portion of the cerebellum. 



FIG. 113. HEAD OF AN EMBRYO CHICK OF THE FOURTH DAY VIEWED 

 AS AN OPAQUE OBJECT : FROM THE FRONT IN A, AND FROM THE SIDE IN B. 

 (Chromic acid preparation.) 



CH. cerebral hemispheres ; FB. vesicle of the third ventricle ; Op. eyeball ; nf. 

 naso-frontal process; M. cavity of mouth; SM. superior maxillary process of F. i, 

 the first visceral fold (inferior maxillary process) ; F. 2, F. 3, second and third 

 visceral folds ; N. nasal pit ; ot. otic vesicle. 



In order to gain the view here given the neck was cut across between the third 

 and fourth visceral folds. In the section e thus made, are seen the alimentary canal 

 a!, the neural canal n.c., the notochord cA, the dorsal aorta AO, and the vertebral 

 veins V. 



The third day, therefore, marks the distinct differentiation of 

 the brain into five distinct parts : the cerebral hemispheres, the 

 central masses round the third ventricle, the corpora bigemina, 

 the cerebellum and the medulla oblongata ; the original cavity 

 of the neural canal at the same time passing from its temporary 

 division of three single cavities into the permanent arrangement 

 of a series of connected ventricles, viz. the lateral ventricles, the 



