HOMOLOGY. 



21 



collapsed roof of tlie skull, there were first seen two hollow 

 sacs, placed symmetrically, with little cerebral substance in 

 their walls ; and between these was found a larger and more 

 tliinly -walled pouch projecting forwards, mesially, from the 

 brain behind." " The mesial pouch when floated out in 

 spirit was then seen to have a considerable cavity. Its mem- 

 branous wall was so thin that it was much damaged in the 

 removal, but the pituitary body adherent to it shows it to be 

 the infundibulum." 



Perception of the homologies above indicated, and re- 

 searches stimulated by the idea, led to thoughts of their 

 bearing upon the following higher generalization. 



At the period of my student's career in Paris the biological 

 mind was exercised by the question of " Unity of Plan " or 

 of Composition " in the Animal Kingdom as exemplified 

 between Articulates and Vertebrates by reversing the position 

 of the former, and turning what was regarded the under or 

 ventral side of the crustacean or caterpillar upward, as shown 

 in fig. 3, so as to correspond with the upper or dorsal side of 

 the Fish or Quadruped. The alleged " Law " was further 

 elucidated, as between Vertebrates and Mollusks, by bending 

 a quadruped so as to bring the pelvis in contact with the 

 nape, and so parallelling it with a cuttlefish — propositions 

 adopted as demonstrative of their " Unity of organic Plan or 

 Composition " by Geoffroy St.-Hilaire. 



To the first of these attempts Cuvier opposed the obvious 

 fact that, though the ganglionic cord of the Articulate might 

 be so brought to the relative position, or place, of the spinal 

 cord of the Vertebrate, yet the chief part of the nervous 

 system, or neural axis, universally recognized as " brain " in 

 both, held opposite relations to the alimentary canal, being 

 above the mouth in the Vertebrate and below the mouth in 

 the upturned Articulate (as is shown in figs. 2 and 3). 



