146 



DEVELOPMENT DURING THE THIRD DAY. 



In the hind-brain, or third cerebral vesicle, the roof of the part 

 which lies nearest to the mid-brain, becomes during the third day 

 marked off from the rest by a slight constriction. This distinction, 

 which becomes much more evident later on by a thickening of the 

 walls and roof of the front portion, separates the hind-brain into the 

 cerebellum and the medulla pblongata (fig. 112 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 mem- 

 brane, 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, subse- 

 quently overhung by the largely-developed posterior portion of the 

 cerebellum. 



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 third ventricle, the 

 iter (with a prolongation into the optic lobe on each side), and the 

 fourth ventricle. 



By the third 

 day the lens of the 

 eye has become 

 formed by an in- 

 vagination of the 

 epiblast, and other 

 changes in the eye 

 have taken place. 

 The external open- 

 ing of the auditory 

 pit is closed before 

 the completion of 

 the third day (fig. 

 1 14, BL) ; and the 

 rudiments of the 

 external parts of 

 the organ of smell 

 have become form- 

 ed as small pits on 

 the under surface 

 of the fore-brain 

 (fig. 11 2, N). Like 

 the lens and the 

 labyrinth of the 

 ear, they are form- 



u 



Fio. 113. HEAD OF AN EMBRYO CHICK OF THE FOURTH DAY 



VIEWED AS AN OPAQUE OBJECT I FROM THE FRONT IN A, AND 



FROM THE SIDE IN B. (Chromic acid preparation.) 



CH. cerebral hemispheres ; FB. vesicle of the third ven- 

 tricle ; Op. eyeball ; /. naso-frontal process ; M. cavity of 

 mouth ; SM. superior maxillary process of F . 1, 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 <tl, the 

 neural canal n.c., thenotochordc/;, the dorsal aorta AO, and 

 the vertebral veins T'. 



