466 C. JUDSON HERRICK 



is found on both sides of this fissure, the ventral limb having been 

 differentiated at the expense of the reptilian primordium hippo- 

 campi. 



DISCUSSION 

 DIENCEPHALON AND TELENCEPHALON 



Since the cerebral hemispheres are formed by a lateral evagi- 

 nation of the walls of the rostral end of the neural tube, a clear 

 understanding of the morphological features of the unevaginated 

 portion of the tube is essential to a correct interpretation of the 

 relations of the parts of the hemispheres. 



In the lower (epichordal) parts of the embryonic brain tube, 

 the massive lateral wall is divided by the sulcus limitans into 

 dorsal and ventral laminae, of which the former (alar or epenceph- 

 alic plate of His) is devoted primarily to the receptive functions 

 and the latter (basal of hypencephalic plate of His) to the effer- 

 ent or effector functions. Correlation tissue of the formatio 

 reticularis type is developed in both laminae, afferent elements 

 predominating in the dorsal and efferent elements in the ventral 

 lamina. Similarly, the descending fibers of the great longitudi- 

 nal conduction paths tend to develop in the ventral lamina and 

 the ascending fibers in the dorsal lamina; but in the upper (pre- 

 chordal) parts of the neural tube, this relation is disturbed by 

 massive correlation centers. 



The absence of peripheral motor nerves rost rally of the mid- 

 brain involves the great reduction of the ventral lamina in this 

 region. Accordingly, the sulcus limitans disappears in the dien- 

 cephalon. In the human embryo (His) it appears to end in the 

 preoptic recess, which would imply that the chiasma ridge and 

 hypothalamus belong tn the ventral lamina. And such indeed 

 has been clearly shown to be the case, though with great secondary 

 distortion due to the absence of the somatic motor nuclei and 

 the invasion of the residual motor coordination tissue by other 

 elements, chiefly of the visceral type (see Johnston, '09, p. 517). 



From the absence of the ventral lamina rostral to the chiasma 

 ridge, it follows that the remaining parts of the diencephalon 



