THE SKELETON OF THE VERTEBRATE HEAD. 123 



tomes. Corresponding, but much more remarkable changes, 

 to which attention has not been hitherto sufficiently directed, 

 accompany the withdrawal from before backwards of the 

 anterior part of the brain from the ethmoidal and pre- 

 sphenoidal sclerotomes. The neural portions of these sclero- 

 tomes assume more or less of the catacentric character they 

 become demicatacentric. The neural chamber of the ethmoidal 

 sclerotome of the mammal, in addition to a portion of the 

 cerebrum proper, lodges its homologous segment of the 

 neural axis. In the bird the neural chamber of this sclero- 

 tome is completely evacuated by the neural axis, which not 

 only leaves it, but withdraws in part also from the pre- 

 sphenoidal. The absence of the anterior extremity of the 

 neural axis from' the neural chamber of the ethmoidal 

 sclerotome is accompanied by the division of that chamber 

 into a right and left compartment by a mesially laminar 

 centrum, the two compartments being occupied by the olfactory 

 capsules. The olfactory lobes in the bird are not only with- 

 drawn from the ethmoidal sclerotome, but retreat to a certain 

 distance backwards in the neural chamber of the presphenoidal. 

 To this extent the chamber becomes catacentric ; but instead 

 of its two resulting compartments being occupied by new 

 structures, having only to transmit the olfactory nerves, their 

 outer walls collapse upon the mesially laminar centrum, and 

 very generally disappear almost altogether, so as to leave the 

 nerves uncovered on the sides of the laminar centrum as they 

 pass forward to the ethmoidal chambers. The neural chamber 

 of the ethmoidal sclerotome of the bird, containing only the 

 olfactory capsules, is so connected with the bones of the face 

 and with the neural arch and centrum of the presphenoidal, 

 as to be more or less movable along with the lower mandible. 

 The ethmoidal in the mammal is thus seen to be the anterior 

 cerebral sclerotome, while in the bird it becomes the posterior 

 facial sclerotome. 



