72 



BRAINS OF RATS AND MEN 



point is the enormous size of the organ of smell and 

 the olfactory reflex centers of the brain (Herrick, 

 1924, chap, xiii and p. 182). A cross-section through 

 the forebrain of a late embryo of this fish shows that 

 its walls contain several separate areas of densely 

 crowded cells (Fig. 7), which Holmgren (1922) and 



hippocampus 



general cort-cx 



hippo com pus 



olfactorq 

 tubercle 



olfaclorij 

 " bulb 



lateral olfactorij 

 nucleus 



medial olfactorLj nucleus 



Fig. 7. — Diagrammatic cross-section through the forebrain of an 

 embryo of the dogfish, illustrating the cellular arrangement. The fields 

 designated as general cortex and hippocampus are regarded by Johnston 

 (1923) as primordia of the corresponding mammalian cortical fields. 



Johnston (1923) think are beginnings of true cortex. 

 These sheets of cells are less evident in the adult, and 

 until more is known of their fibrous connections 

 the question must remain undecided. The lungfishes 

 (Elliot Smith, 1908; Holmgren and van der Horst, 

 1925) show clearer evidence of a poorly developed 

 cortical rudiment in the adult. 



The shape of the cerebral hemispheres of lung- 

 fishes and amphibians (Fig. 8) is much more like that 

 of higher animals than are those of fishes generally, 

 but the internal organization of these brains is much 



