62 



THE SUBDIVISIONS OF THE BRAIN. 



The Optic Ganglia. — The palial overgrowth carried the united parietal eyes 

 over the region of the old stomodaeum, thus helping to form the roof of the third 

 ventricle, and giving rise to the ganglion habenulae and its commissural strands. 



The lateral eye ganglia also united to form a part of the brain roof, but were 

 crowded still farther backward, beyond the tween-brain neuromeres, carrying with 

 them the stomodaeal commissure. The latter then became the anlage of the cere- 

 bellum, and the optic ganghon became the tectum opticum. (Figs. 3 and 46.) 



The optic tracts extend diagonally backward and upward from the optic 

 chiasma to the optic ganglia, and help to form the external lateral walls of the 

 diencephalon; the primitive cerebellar tracts extend diagonally downward and 

 forward, over the inner or ventricular surface of the diencephalon to the lobi 

 inferiores. Thus the location and direction of these important fiber tracts still 



tell the history of the parts in which they term- 

 inate. (Fig. 46, D.) 



The Cheliceral Neuromere. — The an- 

 terior neural portion of the cheliceral neuro- 

 mere is probably in part comparable with the 

 thalamus division of the diencephalon. It con- 

 tains the great masses of association cells going 

 to the hemispheres and cheliceral lobes (Fig. 

 49, ch.l.) and also the cheliceral nerves and 

 ganglia. (Fig. 49, ch.g.) The cheliceral nerves 

 unlike all the other cranial nerves, arise from 

 the median neural surface of the neuromere. 

 They are probably crowded against the cere- 

 bellar commissure by the enlargement and 

 backward migration of the optic lobes. (Figs. 47, 57 and 58.) They are represented 

 in vertebrates apparently by the fourth nerves, which seem to arise from the roof 

 of the brain, between the optic lobes and the cerebellum, although their roots 

 have their origin far forward, on the floor of the midbrain region. The excep- 

 tional location and direction of these nerves in vertebrates, therefore, is in harmony 

 with their exceptional location and direction in arachnids; and the extraordinary 

 resemblance between them affords collateral evidence in confirmation of the ex- 

 planation just given for the origin of the tectum opticum and the cerebellum. 

 Otherwise these pecuHarities of the fourth nerve are inexplicable. 



******* ** 



It will be observed that the whole floor of the vertebrate brain consists of 

 more or less modified neuromeres. Those structures which now form the roof 

 of the brain, such as the palium, ganglion habenulae, optic lobes, and cere- 

 bellum, are not in any way comparable with neuromeres; they have no segmental 

 value, and they now have no genetic relations with the neuromeres over which 



Fig. 50. — Portion of the cerebral cortex 

 of a young Limulus. Camera, Zeiss obj. i6 

 mm., oc. 18. Golgi preparation. 



