ORIGIN OF THE CEREBRAL HEMISPHERES 443 



region are formed not merely by evagination of the lateral wall 

 of the neural tube, but also by thickening of the walls in situ, 

 and especially of the terminal plate, thus separating the terminal 

 part of the common ventricle into two lateral ventricles with a 

 minimal amount of true evagination. The caudal part of the 

 endbrain is much less highly differentiated, especially in the lower 

 tj^pes of elasmobranchs. 



Examination of the development of the cerebral hemispheres 

 of the Amphibia, taking Amblystoma as a type, shows a very 

 different history. Early larvae in Coghill's non-motile stage 

 show no evagination of the walls of the endbrain, but instead 

 the lateral walls are thickened. Immediately external to these 

 thickenings are the developing nasal sacs, the arrangement being 

 similar in principle to that described above for teleosts (fig. 2). 

 In immediately following stages evagination of the hemispheres 

 begins and in the 10-mm. larva, which is an active swimmer, 

 this evagination is well advanced. 



The evagination of the amphibian hemisphere, however, does 

 not begin at the site of the future olfactory bulb, but at the 

 extreme caudal end of the endbrain just in front of the velum 

 transversum. This region, which forms the posterior pole of 

 the hemisphere, is well evaginated before there is any notable 

 outpouching in the more rostral part of the endbrain close to the 

 lamina terminalis which lies adjacent to the nasal sac and which 

 will later form the olfactory bulb. 



A somewhat similar condition prevails in Lepidosiren, as shown 

 by Graham Kerr's illustrations ('02, fig. 10) ; but in this case the 

 peculiarity in question is correlated with the fact that the olfac- 

 tory bulb develops not at the rostral end of the primitive end- 

 brain, but on the dorsal border near its caudal end in immediate 

 contact with the nasal sac. These relations are shown in figure 

 17. In relatively late stages the oKactory bulb shifts its position 

 to become terminal in the hemisphere. Whether this peculiar 

 condition in Lepidosiren is indicative of a similar feature in the 

 ancestors of the Amphibia it would be rash to say; certainly it 

 is possible that the primitive position of the olfactory evagination 



