THE PATH OF DEGENERATION IN ANIMALS 1 8 1 



In Fish The cerebral hemispheres 



and Batrachia. do not cover the region 



of the third ventricle 



from which the eyes arise 



(thalamencephalon). 



In the human 

 embryo (fig. 

 61, A) of the 

 seventh week. 



Same 

 aspect. 



In Reptiles. The hemispheres cover In the human Same 



the thalamenaphalon but ; embryo (fig. aspect, 



leave uncovered the region ! 61, B) of the 



of the optic lobes (mesen- ! middle of the 



cephalon). third month. 



In mammals. 



The hemispheres cover 

 the thalamencephalon, 

 the mesencephalon, some- 

 times the metencephalon 

 (cerebellum and medulla), 

 and the olfactory lobes. 



In the human Same 

 embryo (fig. aspect. 

 61, c) of the 

 fifth month. 



In some mam- The hemispheres 



raals even of smooth. 



higher orders 



(e. g. some 



Hapalidse). 



are In the human Same 

 embryo (fig. aspect. 

 61, D) of the 

 midddle of the 

 fifth month. 



Within such limits, the law of recapitulation 

 may be applied, and the embryonic history of an 

 individual may be considered roundly as a repeti- 

 tion of the essential phases of its ancestral history. 

 We have now to consider how far a reduction by 

 atrophy or by arrest represents a retracing of steps 

 in evolution (fig. 61). 



From this point of view, we may study the 



