Herrick, Morphology of JVcrvoHs Systc?//. 157 



Selachii, which, on the other hand, gives rise to the structure 

 of the prosencephalon in Dipnoii, Amphibia, and Reptilia. 

 It is among these forms that the first indications of the origin 

 of the prosencephalon of higher vertebrates may be sought. 

 The first step toward the higher organization consists in the 

 reduction of the lobus infundibuli and a transformation of 

 the tracts connecting with the caudal parts of the nervous 

 system. The development of the thalami and the reduction 

 of the lobus infundibuli alters the development process of the 

 neural tube in higher vertebrates from the earliest stages on. 

 There appears a diencephalic and a secondary fore-brain, the 

 former having an entirely different significance from the 

 posterior part of the prosencephalon of fishes, for it is the 

 result of accelerated development of a certain dorsal portion 

 of the neural tube which remains undeveloped in fishes. 

 With the gradual development of the thalami and alteration 

 of the rhinencephalon of Selachii, there gradually arise the 

 tegmentum cruris and the pes pedunculi of higher verte- 

 brates. 



" The Thalamus.- — The lateral walls of the lobus infun- 

 dibuli form two round, lateral projections, the lobi inferiores, 

 containing a considerable cavity. The posterior part of the 

 lobe extends into a membranous sac, the so-called saccus 

 vasculosvis. Ventrad, the lobe connects with the bilobed 

 hypophysis. 



"The walls of the lobus infundibuli consist of finely- 

 granular stroma, staining pink with carmine, resembling 

 that of the peripheral portion of the cerebellum. In the 

 peripheral portion this stroma is compact, and it requires 

 high powers to make out a fine fibrous structure parallel to 

 the surface. Two fibre systems are more obvious entad. 

 The fibres of the first run parallel to the surface, those of the 

 second radial to the surface. The latter spring from a layer 

 of tissue clothing the inner surface. This layer is composed 

 of a few series of round cells, much resembling those of the 

 granular layer of the cerebellum. They are, however, some- 



