THE NERVOUS SYSTEM 



545 



The neopallium, by great enlargement, forms most of the hemispheres; 

 the archipaUium develops into the olfactory and hippocampal lobes. 



The wall of the brain consists primarily of a simple columnar epithe- 

 lium, which in the region of the basal and alar plates rapidly becomes 

 many-layered. Soon three strata may be distinguished, an inner epithe- 

 lial or ependyma layer where the most rapid multiplication of cells occurs, 

 a middle relatively thick mantle layer of compact epithelial cells, and an 

 outer marginal layer of nerve fibers. As the hemispheres enlarge, cells 

 migrate from the mantle layer into the marginal zone where they form 

 the cortex, a layer which, lacking in lower vertebrates, appears first in 

 Reptiles. The cortical cells of the definitive hemispheres become arranged 

 in five layers of multipolar neurons. 



Fig. 450. — Median section of brain of pig 15.5 mm. long, showing* flexures of the 

 brain. C, primary flexure; cs, corpus striatum; CP, chorioid plexus of fourth ventricle; 

 h, hypophysis; i, infundibulum; M, mid-brain; A^, nuchal flexure; nc, notochord and 

 vertebrae;' F, pontal flexure; RO, optic recess; T, 'twixt-brain. (From Kingsley's 

 "Comparative Anatomy of Vertebrates.") 



During its early expansion, the surface of the hemisphere is smooth. 

 In the fourth month, fissures arise, and eventually cut the hemisphere into 

 lobes and gyri. The first to appear are those between the olfactory lobes 

 and the pallium. Later, the hippocampal, lateral, and other fissures 

 develop successively. These are followed during the last two months of 

 fetal life by shallower sulci. The chief factor in forming these grooves is 

 the more rapid increase of cortex, compared with that of the underlying 

 white fibrous matter. 



The lumen of each hemisphere persists as the lateral ventricle of the 



adult brain. 



The corpora striata develop as local thickenings of the ventro-lateral 

 walls of the telencephalon. As the dorsal pallium expands into the 

 enlarged neocortex, the thickness of the corpus striatum steadily increases 

 by cell multiplication. Consequently, in a six weeks embryo, each striate 

 body projects into the lateral ventricle as a conspicuous swelUng. Pri- 



