430 



C. JUDSON HERRICK 



tically all of the endbrain, has been effected in so diverse ways 

 as to present one of the most remarkable series of evolutionary 

 changes known in biology. It is to this series of events that I 

 wish to direct attention. 



In embryos of fishes and amphibians shortly after the closure 

 of the neural tube we find a stage of great interest in the analysis 

 of the probable functional factors operating in early stages of 

 the evolution of the cerebral hemispheres. In teleosts at the 

 stage shown in figure 1 the optic vesicles have evaginated from 



NASAL SAC 

 ENDBRAIN 

 OPTIC VESICLE 

 BETWEENBRAIN 



C!i I If i (a 



ENDBRAIN 

 NASAL SAC 

 OPTIC STALK 

 BETWEENBRAIN 

 LENS 



OPTIC VESICLE 

 MIDBRAIN 

 MEDULLA OBLONGATA 



Fig. 1 Surface view of the head end of a 33-hour embryo of the sea-bass 

 (Serranus atrarius) from the dorsal side, illustrating the evaginated optic vesicles 

 and the small (and still solid) endbrain. Modified from H. V. Wilson ('91, fig. 

 146). 



Fig. 2 Surface view of a 62-hour embryo of the sea-bass illustrating the primi- 

 tive endbrain with thickened lateral walls in close contact with the nasal sacs. 

 Modified from H. V. Wilson ('91, fig. 149). 



the neural tube and their cavities communicate with the third 

 ventricle through the hollow optic stalks (precursors of the optic 

 nerves). The endbrain is a very small rudiment. In a later 

 stage, as illustrated in figure 2, the eyes have differentiated 

 further and the endbrain has enlarged somewhat. In each 

 lateral wall of the latter region there is a thickening which repre- 

 sents an accumulation of embryonic nervous tissue from which 

 wiU develop the adult olfactory bulb and the olfactory correla- 

 tion centers lying behind it. There is as yet no evagination of 

 any part of the walls of the endbrain. In later stages the anterior 



