CHAP, x.] THE DEVELOPMENT OF THE CAT. 



357 



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

 the dorsal surface. 



Fig. 160. DIAGRAM OF BRAIN DEVELOPMENT, TO SHOW THE ESSENTIAL RELATIONS OF ITS PARTS. 



A. The brain in its earliest condition of three 



hollow vesicles, the cavity of which is con- 

 tinuous with the wide cavity (d) of the 

 primitive spinal marrow (m). The brain 

 substance forms an envelope of nearly equal 

 thickness throughout. The wall of the 

 anterior end of the first vesicle is the 

 lamina tenninalis (/). 



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



veloped the pineal gland (jm) above, and 

 the pituitary body ( pt) below, the cerebral 

 hemisphere (cr) are beginning to bud forth 

 from the front part of the first vesicle. 



C. The cerebrum is here enlarged, and has begun 



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

 cavity of the cerebrum (or incipient lateral 

 ventricle) communicating with that of the 

 olfactory lobe in front, and with that of the 

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

 ventricles) behind. The latter communica- 

 tion takes place through the foramen of 

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

 vesicles are becoming unequal in thickness, 

 and the cavity of the middle vesicle (I) is 

 becoming reduced in relative size, as the 

 iter a tertio ad quartiim ventriculum. 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 corpora quadrigemina (</), and the 

 roof and floor of the third cerebral vesicle (c) 

 have thickened into the cerebellum (cb) and 

 pons Varolii(jw). 



D. The figure shows the cerebrum more en- 



larged, with its cavity bifurcating into the 

 prolongations (the cornua). The 1'ornix has 

 come to look backwards, and two lines 



(below the figure 3) indicate its prolongation 

 downwards and backwards into the corpora 

 mammillaria (ma). 



E. Here the cerebrum is still more enlarged, 

 extending backwards over the corpora 

 quadrigemina and part of the cerebellum. 

 The fornix is shown bordering the descend- 

 ing cornu (dc) of the lateral ventricle, and 

 extending into the temporal lobe (tl). 



a. Fore-brain. 



oc. Anterior eornu. 



b. Mid-brain. 



c. Hind-brain. 

 cb. Cerebellum. 

 cr. Cerebrum. 



dc. Descending cornu. 



d. Cavity of the medulla. 

 /. Fornix. 



fir. Corpora quadrigemina. 



1. Iter a tertio ad quartum ventriculum. 

 m. Medulla oblongata. 



ma. Corpora mammillaria. 



o. Olfactory lobe. 



pv. Pons Varolii. 



pn. Pineal gland. 



pt. Pituitary body. 



f. Crura cerebri. 



t. Lamina terminalis. 



tl. Temporal lobe of cerebrum. 



x. Space enclosed between the backwardly- 

 extended cerebrum and the much narrowed 

 roof of the third ventricle, which has now 

 become the velum interpositum. 



2. Lateral ventricle. 



3. Third ventricle. 



4. 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-brain. To this succeed two other 



