REC.IONAL ANALYSIS 55 



bulb, and amygdala is less secure and awaits confirmation or correc- 

 tion. 



The anterior olfactory nucleus is a zone of relatively undifferenti- 

 ated cells interpolated between the bulbar formation and the more 

 specialized areas posteriorly of it (figs. 6, 86B and C, 105, 109; '27, 

 p. 288). In higher animals much of this tissue seems to be specialized 

 and added to the adjoining fields ('24(/). A very large proportion of 

 the fibers of the olfactory tracts, arising from both the bulbar forma- 

 tion and the anterior nucleus, are assembled in a dense superficial 

 sheet of fibers in the medial sector of the anterior olfactory nucleus, 

 which I have named the "fasciculus postolfactorius" (figs. 5, 100, 

 105, 110, /./JO.). Here these fibers take a vertical course and then are 

 distributed to all the olfactory tracts. In chapters vii and xix there 

 is further discussion of the significance of the olfactory system in the 

 morphogenesis of the hemisphere. 



17. PALXiIUM 



The pallial part of the hemisphere can be distinguished from the 

 stem part, though there is no laminated cortex. There are three sec- 

 tors (figs. 96-99) — the dorsomedial primordium hippocampi {p.hip.), 

 the dorsolateral primordium piriforme {p.pir., or nucleus olfactorius 

 dorsolateralis, nuc.oLd.L), and between these a dorsal sector of un- 

 certain relationships (p.p.d.). The gray, as elsewhere in these brains, 

 is confined to a thick periventricular layer except in the hippocampal 

 sector, where the cell bodies are dispersed through the entire thick- 

 ness of the wall and are imbedded in dense neuropil. This is evidently 

 a first step toward differentiation of superficial cortex. The homolo- 

 gies of the hippocampal and piriform sectors with those of mammals 

 are clear, as shown by substantially similar nervous connections. 

 Further discussion will be found in chapter vii and the references 

 there given. 



THE COMMISSURES 



Throughout the length of the central nervous system all parts of 

 the two sides are broadly connected by systems of commissural and 

 decussating fibers. These are in two series, dorsally and vent rally of 

 the ventricles. Their composition is summarized in chapter xxi, with 

 references to more detailed descriptions. In the aggregate they make 

 provision for the co-ordinated action of the motor organs on right and 

 left sides of the body. 



