204 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



pre-sphenoids are developed in the trabeculae, the position of the 

 stalk of the stomodaeal process is seen in the later months of 

 foetal life between these two bones and forms the canalis cranio- 

 pharyngeus (Fig. 3). 



Pineal Body (see Fig. 167) grows as a hollow bud from the 

 dorsal plate of the hinder part of the fore-brain during 

 the 6 th week. In fossil reptiles and in some forms still 

 living it forms a median eye which perforates and appears on 

 the dorsum of the head between the parietal bones. It differs 

 from the lateral eyes which grow from the third ventricle as the 

 optic vesicles in this, that it produces the lens as well as the 

 retina and optic stalk. The retina is inverted — i.e. the apices of 

 the rods and cones point towards the vitreous chamber. The 

 ganglion of the habenula, situated on the dorsal and inner aspect 

 of the optic thalamus, represents its terminal ganglion. In man 

 and mammals its development is arrested at an early stage. It 

 produces a number of diverticula which are filled up by a pro- 

 liferation of the cells which form the walls of the diverticula. In 



pia mater 



cereb. uesicle 

 posit, of corp. call, 

 caudate nuc. 



tat vent, 

 tenth, nuc. 

 to for. Monro 



desc. horn. 



cereb. uesicle 

 arcuate fis. 



choroid plex. 

 eaud. nuc. 

 uel. interposit 

 lentic. nuleus 



optic thalamus (alar lamina) 



3rd ventricle 



Fig 168.— Transverse Section of the brain of a Human Foetus at the commencement 

 of the 3rd month to show the Cerebral Vesicles overlapping the Thalamence- 

 phalon (schematic). 



man it appears to be merely vestigial. It lies in the velum 

 interpositum, which is forced down on it by the growth back- 

 wards of the cerebral hemispheres. 



