DEVELOPMENT OF THE BEAIN AND GANGLIA. 51 



masses are separated by a cleft from a more dorsal segment, the Pallium, or 

 mantle, whose walls thicken relatively late. It is interesting to note the 

 primitiye relations which manifest themselves here. In all vertebrates the 

 basal ganglia — corpora striata and olfactory lobes — are developed, but only 

 among the higher vertebrates does the mantle reach a noteworthy develop- 

 ment. In Petromyzon and in bony fishes the mantle remains a simple 

 epithelial wall throughout life. But the Pallium is that portion of the brain 

 which later bears the cortex cerebri, and is, therefore, the organ on whose 

 development all higher psychical life depends. The Pallium of the stur- 

 geon, for example, remains through life as thin as it is represented in the 

 four months' embryo (Pig. 18). 



Vierhugfl- 



Cerfbellun, 



Hvpophusenanlye. 



Fig. 21. — Longitudinal section of head of a four-and-a-half day chick. The 

 five brain-vesicles are fairly well developed. YorderMrnMMe, Cerebral cavity. 

 ZwisdhenMrnhohle, Thalamencephalic cavity, or third ventricle. Mittlehirn- 

 ftoftZe, Aquaeductus. HinterhirnJibMe, Cerebellar cavity. NachhirnJiohle, Cavity 

 of medulla. The last two together form the fourth ventricle. HypopTvyserwmlage, 

 Fundament of the hypophysis. YierMgle, Corpora Quadrigemina. (After von 

 Mihalkovics.) 



The hemispheres are, in mammals, most highly developed, and are also 

 here best studied. Their development should be, therefore, especially de- 

 scribed at this point; while those parts of the brain which lie posterior to 

 the cerebrum may better be described after we are acquainted with the 

 brains of lower animals where the thalamus, and midbrain, and even the 

 cerebellum show especial structural forms, which are either insignificant or 

 quite lost in mammals. The roof of the Thalamencephalon remains through 



