DEVELOPMENT OF THE ENCEPHALON. 961 







slightly bent downwards over the anterior extremity of the notochord, and 

 this is the commencement of the cranial flexure. The cavity in the interior 

 of each of the two vesicles of the cerebral hemispheres originating in the 

 manner just described, constitutes the lateral ventricle, whilst the cavity of 

 the fore-brain from which they were budded off remains as the cavity of the 

 third ventricle, or, as Foster and Balfour term it, the " 'tween brain." On 

 the summit of the 'tween-brain the rudiment of the pineal gland makes its 

 appearance, whilst the floor is produced into the " iufundibulum," which 

 joins the pituitary body. The roof of the mid-brain becomes developed into 

 the corpora bigemina or optic lobes in Birds, but into the corpora quadrigem- 

 ina in Mammals ; its floor is formed by the crura cerebri, and its cavity be- 

 comes reduced to the Her a tertio ad quartum ventriculum. In the mid-bruin, 

 or third cerebral vesicle a constriction appears in the course of the third day, 

 which indicates a division into the cerebellum in front, and the medulla ob- 

 lougata behind. The fundamental parts of the brain are thus fairly mapped 

 out in Birds on the third day. In Mammals the series of vesicles above de- 

 scribed are distinctly visible at about the sixth week, and in the small pro- 

 portion which the Cerebral Hemispheres bear to the other parts, in the ab- 

 sence of convolutions, in the deficiency of commissures, and in the general 

 simplicity of structure of the whole, there is a certain correspondence be- 

 tween the brain of the Human embryo at this period and that of a Fish ; 

 but the resemblance is much stronger between the fcetal brain of the Fish 

 and that of the Mammal ; indeed, at this early period of their formation, the 

 two could scarcely be distinguished ; and it is the large amount of change 

 which the latter undergoes, as compared with the former, that causes the 

 wide dissimilarity of their adult forms. At about the 12th week, we find 

 the Cerebral Hemispheres much increased in size, and arch ing-back over 

 the Thalami and Corpora Quadrigemina (Fig. 363); still, however, they 

 are destitute of convolutions, and are imperfectly connected by commissures; 

 and there is a large cavity yet existing in the Corpora Quadrigemina, which 

 freely communicates with the Third Ventricle. In all these particulars, 

 there is a strong analogy between the condition of the brain of the Human 

 embryo at this period and that of the Bird. Up to the end of the 3d mouth, 



..... d 



Brain of Human Embryo at twelfth week : A, seen from behind ; B, side view ; c, sectional view ; a, 

 corpora quadrigemina; ft, ft, hemispheres; d, cerebellum; e, medulla oblongata ; /, optic thai ni us ; g, 

 floor of third ventricle ; I, olfactory nerve. 



the Cerebral Hemispheres present only the rudiments of anterior lobes, and 

 do not pass beyond that grade of development which is permanently char- 

 acteristic of the Marsupial Mammalia, the Thalami being still but incom- 

 pletely covered in by them. During the 4th and part of the 5th months, 

 however, the middle lobes are developed from their posterior aspect, and 

 cover the Corpora Quadrigemina; and the posterior lobes, of which there 

 was no previous rudiment, subsequently begin to sprout from the back to 

 the middle lobes, remaining separated from them, however, by a distinct 

 furrowj even in the brain of the mature foetus, and sometimes in that of 

 older persons. In these and other particulars, there is a very close corre- 



