649 



of the lemniscus system and also optic fibers in the corpus geniculatum 

 laterale, which is a differentiation of its lateral border. This part is 

 attenuated in front but is much larger in the frog and in Amniota. 

 The pars dorsalis thalami is the direct forward continuation of the 

 dorsal lamina of the neural tube of the midbrain region ; but the pars 

 ventralis thalami does not bear this simple relation to the ventral 

 lamina of the midbrain, for as shown by Fig. 3, it lies chiefly dorsal 

 to the sulcus limitans. Nevertheless its position and its fiber connec- 

 tions show it to be functionally and morphologically related to the 

 primary ventral lamina, though this relationship is indirect. 



The suppression of peripheral motor nerves in front of the midbrain 

 results in the great reduction of the motor ventral lamina of the 

 neural tube in the diencephalon. Nevertheless, a certain amount of 

 motor correlation tissue of the formatio reticularis type is preserved 

 here, and rostral to the sulcus limitans similar motor correlation tissue 

 is differentiated to form the pars ventralis thalami. It in fact becomes 

 the avenue of somatic motor correlation and discharge for the pars 

 dorsalis thalami and the telencephalon. In a similar way the hypo- 

 thalamus and preoptic nucleus below the sulcus diencephalicus 

 ventralis become centers for olfactory and visceral motor correlation 

 and discharge, 



Johnston has clearly shown (Journal of comparative Neurology 

 and Psychology, Vol. 19, No. 5, Nov., 1909) that the usage of His 

 must be followed in the definition of the telencephalon, that is, it is 

 the terminal part of the primary neural tube, including the secondarily 

 evaginated hemispheres; and in early embryos and lower vertebrates 

 particularly it consists of a telencephalon medium and the paired 

 cerebral hemispheres which arise as lateral evaginations from the 

 walls of the primordial telencephalon medium. The evagination of 

 the cerebral hemispheres undoubtedly began by the enlargement of 

 the primary olfactory centers. The longitudinal subdivision of the 

 rostral end of the neural tube into four functionally defined laminae 

 on each side, as seen in Figs. 2 and 3, was certainly antecedent to the 

 completion of the evagination of the hemispheres as they are found 

 in the Amphibia. Accordingly, during the process of this evagination 

 these four laminae were carried out into each hemisphere, where their 

 positions were determined by the mechanical necessities of the process 

 of lateral evagination. This, in fact, can readily be shown to be the 

 case. 4s we follow a series of transverse sections of the amphibian 

 brain forward, the roof plate and floor plate converge into the lamina 

 terminalis, where, of course, they end. The four massive columns on 



