NERVOUS SYSTEM OF VERTEBRATES. 



79 



- / 



of the former, if any, is unknown. The latter seems to 

 be a useless remnant of a once useful eye in the top of 

 the head. In some lizards it 

 still retains the structure of an 

 eye (Fig. 47). 



Embryonic compared with the 

 Taxonotnic Series. — It is a most 

 significant fact that the brain 

 of the embryo of man passes 

 through all these stages: (1) 

 The earliest condition of the 

 human brain is seen in Fig. 48. 

 This can be compared with the 

 brain of only the very lowest 

 fishes. It may therefore be 

 called the subfish stage. It does 

 not yet contain a cerebrum. 

 (2) Then the cerebrum grows 

 out of the thalamus, but is yet inferior in size to the 

 optic lobes (Fig. 49). This may be called the average 

 fish stage. (3) Then the cerebrum grows until it is the 

 largest of the series, but covers nothing as yet (Fig. 

 50). This may be called the reptile stage. (4) Then it 

 begins to cover the optic lobes (Fig. 51). This corre- 



FlG. 47. — Parietal eye of Hat- 

 teria (after Spencer) : /, lens ; 

 v, vitreous humor ; r, ret- 

 ina ; o , optic nerve. 



Fig. 48. — Sub-fish stage : tk, thalamus ; ol, optic lobe ; m, medulla. 



sponds to the bird stage. (5) Then it covers the whole 

 of the optic lobe. and. encroaches on the olfactory lobe 

 7 



