I III: BRAIN 



329 



thin, dorso-lateral roof of the myelencephalon has been removed). The 

 telencephalon is a paired structure. In the figure, its right half projects 

 cranial to the primitive median wall of the fore-brain, which persists as 

 the lamina terminalis (cf. Fig. 342). The floor of the telencephalon is 

 greatly thickened caudally as the anlage of the corpus striatum. A slight 

 evagination of the ventral wall of the telencephalon, just cranial to the 

 corpus striatum, marks the anlage of the rhinencephalon. The remaining 

 portion of the telencephalon forms the pallium, or cortex, of the cerebral 

 hemispheres. The paired, cavities of the telencephalon are the lateral 

 (first and second) ventricles, and these communicate through the interven- 



Cerebral peduncle. 



Hypothal, 

 Epithalamus 

 Thalamu 

 Diencephalon- 



Cerebral aqueduct 

 l^Mesencephalon 



Pallium 

 Telencephalon-.. 



^.Rhombencepltalic isthmus 



Cerebellum 



{ ; Metencephalon 



Rhomboid fossa 



Myelencephalon 



Rhinencephaltn I Corpus striatum Pans 

 Lamina terminalis 



Spinal cord 



FIG. 332. Brain of a 13.6 mm. human embryo in median sagittal section (after His in Sobotta). 

 I, Optic recess; 2, ridge formed by 3, the optic chiasma; 4, infundibular recess. 



tricular foramina (of Monro) with the cavity of the diencephalon, the third 

 ventricle. The cavities of the olfactory lobes communicate during fetal 

 life with the lateral ventricles and were formerly called the first ventricles. 

 The crossing of a portion of the optic nerve fibers in the floor of the 

 brain forms the optic chiasma, and this, with the transverse ridge produced 

 by it internally, is taken as the ventral boundary line between the telen- 

 cephalon and diencephalon (Pig. 332). A dorsal depression separates 

 the latter from the mesencephalon. The lateral wall of the diencephalon 

 is thickened to form the thalamus, the caudal and lateral portion of which 

 constitutes the metathalamus . From the metathalamus are derived the 

 geniculate bodies. In the median dorsal wall, near the caudal boundary 

 line of the diencephalon, an outpocketing begins to appear in embryos of 



