DEVELOPMENT OF THE BRAIN. 61 



general sustentacular substance of both white and grey matter is probably derived 

 from the spongioblasts, and is therefore, like the nerve-cells themselves, of epiblastic 

 or gin. 



DEVELOPMENT OP THE BRAIN. 



We have already traced the development of the cephalic part of the neural tube 

 as far as the formation of the primary cerebral vesicles. These, which are at first 

 three in number (fig. 40), become subdivided so as to form five in all, which may 

 be termed in succession from before back, the first, second, third, fourth, and fifth 

 secondary vesicles. Of these five parts the first two, which represent the cerebral and 

 thalamic parts of the future brain (third ventricle), are derived from the first primary 

 vesicle, and the last two, the cerebellar and bulbar parts (fourth ventricle), from the 

 third primary vesicle, while the third, middle, or quadrigeminal part, represents the 

 undivided second primary vesicle (Sylvian aqueduct). These relationships, as well 

 as the several parts of the brain which are eventually respectively formed in connec- 

 tion with the vesicles, are shown in the subjoined table. 



I Anterior end of third ventricle, fora- 

 mina of Monro, lateral ventri- 

 cles, cerebral hemispheres, olfac- 

 tory bulbs and tracts, corpora 

 striata, corpus callosum, fornix. 



Second secondary vesicle ( "? ltr j? le . ?P . r . f and 



** 



II. Middle primary vesicle (" Third secondary vesicle I Aqueduct of Sylvius, corpora quad- 

 or mid-brain \ (mesencephalori) ( rigemina, crura cerebri. 



III. Posterior primary vesi- 

 cle or hind-brain 



( Fourth secondary vesicle 



(epencephalori) \ Fourth ven- 



Fifth secondary vesicle 

 (jnetcnceplialori) 



tricle 



Cerebellum. Pons. 



Medulla oblongata. 



The first and most striking change which occurs in the primary brain is the 

 outgrowth on either side of the first primary vesicle of a hollow protrusion (primary 

 optic vesicle), which becomes developed eventually into optic nerve and retina 

 (fig. 68). The changes which it undergoes in the formation of these structures will 

 be considered when the development of the eye is dealt with ; suffice it for the 

 present to say that the free hollow communication (optic stalk), which at first exists 

 between the forebrain and optic vesicle, becomes gradually narrowed and at length 

 obliterated, and that as development proceeds, the connection of the optic stalk 

 becomes relatively shifted backwards, so that when the anterior part of the fore-brain 

 is distinct from the posterior part, or thalamencephalon, the optic vesicle is connected 

 wholly with the latter, a relationship which is maintained permanently, although 

 partially obscured afterwards by the later connection which is formed between the 

 optic tract and the mid-brain. Subsequently another pair of hollow outgrowths 

 sprouts from the fore-brain, and these rapidly extend forwards, laterally and back- 

 wards ; they form the vesicles of the cerebral hemispheres. From the roof of the fore- 

 brain (second vesicle) a median hollow protrusion grows upwards and forwards for a 

 certain distance towards the vertex, and from the floor of the same vesicle another 

 somewhat similar protrusion passes downwards and backwards towards the roof of 

 the mouth. The former is the rudiment of the pineal gland, the latter of the 

 infundibulum, which becomes involved in the formation of the pituitary body. 



The principal parts of the brain appear as thickenings in different parts of the 



