MORPHOLOGY OF THE FOREBRAIN 493 



This is the usage adopted by Sterzi and clearly illustrated in 

 his descriptions of the brains of cyclostomes and selachians (see 

 especially 'C9, pp. 694, ff.), and also by Tandler and Cantor ('07) 

 in their description of the development of the gecko brain. The 

 unevaginated portion of the first segment of the neural tube and 

 its adult derivatives (ventriculus impar telencephali,aula,recessus 

 preoptions, lamina terminalis, etc.), then, should be called the telen- 

 cephalon medium, while the hen. isphaeria cerebri are the evaginated 

 portions of the telencephalon, as in the classical usage. 



This is sound morphology from the standpoints of both em- 

 bryology and comparative anatomy. The relation of the hemi- 

 sphere to the telencephalon medium in its earlier stages (both 

 embryonic and phylogenetic) is somewhat sin ilar to that of the 

 retina to the primordial neural tube in the next following segment. 

 In both cases a peripheral sense organ (nose, eye) has called forth 

 in the neural tube massive primary terminal nuclei, whose further 

 enlargement was possible only by an evagination of the wall of 

 the primary tube. An analogous evagination is found in the vagal 

 lobes of cyprinoid fishes, where the hypertrophied gustatory 

 apparatus has called forth a similar response in the medulla 

 oblongata. In the case of the eye the peripheral receptive cells 

 are included in the evaginated tissue (rods and cones of the 

 retina) ; in the nose the primary receptive cells are independently 

 differentiated peripherally in the olfactory placode. In both 

 cases the centers containing the neurones of the second order are 

 completely evaginated and are located in the retina and olfactory 

 bulb respectively. 



In the rhinencephalon the secondary and tertiary olfactory 

 grey are directly continuous with that of the unevaginated pri- 

 mordium and this with diencephalic grey centers farther back. 

 The evaginated portion of the olfactory correlation tissue receives 

 successively more afferent tracts from non-olfactory centers far- 

 her back and by the associations here set up the cerebral cortex 

 was gradually elaborated. 



It thus appears that, while in cyclostomes and some other 

 lowly vertebrates the hemispheres and primordial telencephalon 

 are related to each other in much the same way as are the primary 



THE JOURNAL OF COMPARATIVE NEUROLOGY AND PSYCHOLOGY, VOL. 20, NO. 5. 



