CnAP. XIX.] DURING UTERINE LIFE. 835 



away till mere membrane is left — the 'velum interpositum ; ' at the 

 posterior and upper margin of this ventricle the Phieal hody {'pl) 

 appears, while its floor is prolonged into the infandihuhAm, which 

 subsequently comes into connection with the Pituitary hody {p t). 



But at a very early period, and before the above described parts 

 are distinguishable, an outgrowth (cr) buds on each side from the 

 Anterior Swelling. These outgrowths, which are at first directed 

 downwards and forwards, are hollow, and each communicates with 

 the Third Ventricle through an aperture known as the ' foramen 

 of Munro.' Subsequently these outgrowths undergo an enormous 

 development and constitute the two Cerebral Hemispheres, while 

 their contained cavities persist as the ' Lateral Ventricles,'' and the 

 Corpora Striata develop within them. From each embryo Hemi- 

 sphere another hollow bud-like outgrowth develops anteriorly, and 

 these (o) constitute the Olfactory Lobes and their peduncles. 



From the point of view of its developmental history, therefore, 

 the entire brain is capable of division into three principal parts : — 

 (1.) The Fore-brain, consisting of the Olfactory Lobes, the Cere- 

 bral Hemispheres, and the parts surrounding the Third Yentricle; 

 (2.) the Mid-brain, consisting of the Corpora Qnadrigemina, and 

 the Crura Cerebri; (3.) the Hind-Brain, consisting of the Cere- 

 bellum, the Pons Varolii, and the Medulla oblongata. Of these 

 principal parts, the Fore-brain admits of subdivision into three dis- 

 tinct segments, {a) Olfactory, (&) Hemispherial, and (c) Thalamary; 

 and tlie Hind-brain into two segments, (a) Cerebellar, (b) Oblongate. 

 The Mid-brain needs no further subdivision. This classification, 

 given some years ago by Huxley, has the merit of simplicity when 

 compared with other rather cumbrous nomenclatures now in 

 vogue.* 



In fig. 122 the beginnings of these six principal brain segments 

 are pretty plainly indicated by the parts to which the following 

 letters of reference are attached: — o, cr, a, b, c, m. 



After this preliminary statement, a more detailed 

 account may now be given of the changes undergone by 

 the primitive nerve tube with its cephalic swellings, with 

 the view of giving the reader some notions as to the 

 order and times of occurrence of the several changes. 



* See Gegenbauer's " Elements of Compar. Anat." (Engl. Trans.) 

 p. 503. 



