CRANIAL FLEXURE. FOKE-BRAIN. 69 



This part becomes greatly developed and produced forwards 

 into a large vesicle, the front part of which soon becomes 

 marked off as the rudiment of the cerebrum or prosencephalon. 

 This forward growth is shown by the subsequent relations of the 

 optic nerve to have taken place behind (in the original position of 

 the parts) the attachment of that structure {optic chiasma) to the 

 cerebral roof (Fig. 113). If this interpretation of the complex em- 

 bryonic growths is correct, it would appear that the cerebrum is 

 derived from a dorsal extension of the original fore-brain just behind 

 the point of origin of the optic nerves, and that the olfactory nerves 

 which are developed from the front end of the cerebrum and are 

 usually described as the first pair of cranial nerves, are in reality 

 the second, the optic nerves being anterior to them in position. 

 The optic nerves then are attached to the roof of the original 

 fore-brain at about halfway between its front and hind ends. 

 A ganglionic mass is formed on each side at this point in the 

 side walls of the fore-brain ; these great ganglia are the optic 

 thalami and lie in the adult brain at the side of the third ven- 

 tricle, constituting the chief bulk of the thalamencephalon. 

 Immediately in front of the optic thalami two great ganglionic 

 developments are formed in the ventral wall of the cerebral out- 

 growth : these are the corpora striata found in the adult on the 

 floor of the second ventricle or, if it is divided, of the lateral 

 ventricles. 



The front end of the cerebral outgrowth also gives rise to 

 nervous tissue of the olfactory lobes. 



We now come to the roof of the reconstituted fore-bratn, after 

 the cerebral outgrowth has been formed.* Tliis roof is divided 

 into two parts by the velum transversum (see below). Of these 

 the posterior part, the part overlying the third ventricle, remains 

 in all Vertebrates almost entirely in an epithelial condition. It 

 gives rise by its posterior part to the epiphysis or pineal body. 

 To this we shall return shortly. The anterior part, the part 

 belonging to the cerebral rudiment, is called the pallium. It is 

 marked off from the posterior part by a transversely directed 

 fold of the epithelial roof. This fold dips down into the ventricle 

 at the junction of the thalamencephalon and prosencephalon 

 and encloses between its two laminae a vascular development 

 of the pia mater, which is always present and gives rise in the 

 * Minot, American Journal of Anatomy, 1, 1901, p. 81. 



