MORPHOLOGY OF THE FOREBRAIN 467 



and telencephalon belong to the dorsal lamina, this lamina curv- 

 ing around from the dorsal to the ventral surface, so as to form 

 the whole of the rostral end of the lateral wall of the neural tube 

 at the time of the closure of the neuropore (cf . the diagram of the 

 structure of a hypothetical vertebrate ancestor, fig. 72). 



The optic vesicle is evaginated from the dorsal lamina at a very 

 early age, this process often beginning before the closure of the 

 neural tube. After the closure of the tube, a similar evagination 

 occurs from its rostral end to form the cerebral hemisphere. 

 The optic evagination does not involve the extreme dorsal part 

 of the dorsal lamina, but includes tissue ventral to the epithalamus 

 and rostral to the thalamus (cf. fig. 72). 



The withdrawal of this part of the massive lateral wall into the 

 optic vesicle leaves an area at the site of the future di-telencephalic 

 fissure (margo thalamicus, fig. 82, 2-2) relatively free from pro- 

 liferating nuclei. The origin of this fissure, which is limited to 

 the dorsal part of the wall, is explained by Johnston ('09, p. 516) 

 as due to the evagination of massive tissue from this region into 

 the optic vesicle. 



The telencephalon of all existing vertebrates consists of a prim- 

 itive unpaired vesicle, the telencephalon medium, which is a rem- 

 nant of the first segment of the primitive neural tube, and of 

 secondarily evaginated hemispheres. As we ascend the phylo- 

 genetic series, the hemispheres develop progressively and the 

 median vesicle regressively 



Comparative considerations suggest that in the earliest phylo- 

 genetic stages the evagination of the cerebral hemispheres included 

 only the primary and secondary olfactory centers. The olfactory 

 bulb is, doubtless, the oldest part of the hemisphere. All of its 

 other elements have entered subsequently by the enlargement of 

 the original evagination to include adjacent parts of the wall of 

 the neural tube and the further differentiation of these parts in 

 situ. A part of the primordial secondary olfactory center, how- 

 ever, remains permanently in the telencephalon medium of all 

 vertebrates, viz., the preoptic nucleus; and in higher vertebrates 

 the secondary olfactory fibers for this nucleus are partially or 

 wholly replaced by fibers of the third or higher orders. 



