CHAP. X.] THE DEVELOPMENT OF THE CAT. 



357 



groove and tho approximation of the roof of the canalis centralis to 

 the dorsal surface. 



Fig. 100.— DiAGRAii Of Beain' Development, to show the essential relations of its PAr.Ts. 



A. Tlie hrain in its earliest oomlitinu of tliree 



liollow vesicles, tlie cavity of wliicli is coii- 

 timidus with tlie wide cavity ('/) of the 

 iwiiiiitive siiinal marrow (m). The brain 

 suhstanee forms an envelo])eof nearly equal 

 thickness tlironghout. The wall of the 

 anterior end of the first vesicle is the 

 lamina terminalis {t). 



B. Here the first Aesicle or fore-brain has de- 



veloped the iiineal gland (pn) above, and 

 the pituitary body (pt) below, the cerebral 

 lieinisphere (c/) are beginning to bud forth 

 from the front ])art of the first vesicle. 



C. The cerebrum is here enlarged, and has begun 



to bud out into the olfactoiy lobes (o). The 

 cavity of the ceieljrum (or incipient lateral 

 ventiiele) communicating with that of the 

 olfactory lobe in front, and with that of the 

 first cerebral vesicle (i.e., with the third 

 ventricles) behind. Tlie latter communica- 

 tion takes place through the foranien of 

 Monro (/<») . The walls of the three cerebral 

 vesicles are becoming unef|ual in thickness, 

 and the cavity of the niiildle vesicle (h) is 

 becoming reduced in relative size, as the 

 iter a terfio ail quartuni ventfiviiltiui. The 

 thickened upper part of the cerebrum (/) is 

 what is to become the fornix. The thickened 

 roof of the middle vesicle has divided into 

 the corjiora quadrigemina (</), and the 

 roof and fioor of the third cereliral vesicle (c) 

 have thickeneil into the cerebellum (c6) and 

 pons Varolii (pi). 



D. The figure shows the cen^bnim more en- 



larged, with its cavity bifurcating into the 

 prolongations (the cornua). The forni.x; has 

 come to look backwards, and two lines 



(below the figure 3)inilicate its jirolongatioii 

 downwards and backwards into the corpora 

 niammillaria (ma). 

 E. Here the cerebrum is still more enlarged, 

 extending liackwards over the corpora 

 quadrigemina and jiait of the cerebellum. 

 The fornix is shown bordering the descend- 

 ing eomu ('7c) of the lateral ventricle, and 

 extending into the temi>oral lobe (tl). 



a. Fore-brain. 



ac. Anterior coruu. 



b. Mid-lirain. 



c. Hind-brain. 

 ch. Cerebellum. 

 ct: Cerebrum. 



f/o. Descending comu. 



d. Cavity of the medulla, 

 /. Fornix. 



(/. Corpora quadrigemina. 



i. Iter a tertio ad quartum ventriculum. 



m. Medulla oVilongata. 



ma. C'or])ora mammillaria. 



0. Olfactoi-y l<ibe. 



jic. Pons Varolii, 



'pn. Pineal gland. 



pt. Pituitary iKidy. 



/■. Ciura cerebri. 



f. Lamina terminalis. 



tl. Temporal lobe of cerebrum. 



X. Space enclosed between the baokwardly- 

 extended cerebrum and the much narrowed 

 roof of the third ventiiele, which has now 

 become the velum interpositum. 



2. Lateral ventricle, 



;i. Third ventricle, 



i. Fourth ventricle. 



The BRAIN is first indicated by the expansion of the pre-axial 

 end of the medullary groove into what becomes the first or most 

 anterior cerebral vesicle or fore-hrain. To this succeed two other 



