Functional fai tors in llu- niorpliolojiy of thc fnrchrain of fishes 199 



rius lateralis distribute freely throughout the whole extent of the dorso- 

 lateral sector of the hemisphere, including the pyriform pallium of 

 Soderberg. 



In Amphibia the entire dorso-lateral sector of the hemisphere is part 

 of the nucleus olfactorius lateralis. In mammals the caudal end of the 

 corresponding región is greatly enlarged by the ditíeientiation within it 

 of pyriform cortex. This región, therefore, cuts across the pallial border 

 with no clearly defined boundary between its pallial and subpallial parts. 

 It is transitional in structure and function throughout its phylogenetic 

 history and it furnishes a striking illustration of the fact that the brain is 

 not a rigid mosaic of morphological units which were laid down in the 

 primordial vertébrate and thereafter preserved inviolate. 



Soderberg takes exception to my statement (1 92 1, p. 443) that the 

 evagination of the urodele hemisphere begins at the (morphologically) 

 extreme caudal end of the telencephalon dorsally, adjacent to the velum 

 transversum. However xincredible» this may appearto her, it is never- 

 theless true. I have a considerable series of wax models of larval Am- 

 blystoma brains, including several at earlier stages than the youngest 

 stage of Tritón which she considers, and the initial evagination is clearly 

 at the site of the future posterior pole of the hemisphere. This lateral 

 movement attains considerable dimensions before any other part ot the 

 telencephalon is notably evaginated and it involves the entire pallial área. 

 Three early stages in thjs process of evagination are figured by llurr 1. 

 On account of the strong head flexure at this age, the site of the initial 

 evagination is at the anatomical dorso-rostral border of the telencephalon 

 (cf. Soderberg's figure 23 of an older stage of Tritón); but that this bor- 

 der is the future posterior pole of the hemisphere is evident from study 

 of its subsequent history. 



Kuhlenbeck - in a series of papers has developed a very original 

 scheme of forebrain morphology which is carried through the vertébrate 



' ]}urr, H. S. 1922. The carlv drvclopmcnt oí the cerebral hemispheres in 

 Amhlystoma. Jour. Comp. Nenr., vol. .í4, pp. 277-301. 



- Kuhlenbeck, H. 192 1. «Zur Morplioloi^ie des Uro(lelenvorclcrhinis.> Joi. 

 Zcils. f. Xat., Bd. 57, pp. 403-490. 



1921. vZur Histologie des Anurcnpalliunis.* Anal. An:., U<\. 54, pp. 280-2S5. 



1921. «I)ie Regionen des Anurenvorderhirns.» AnaL An:., Bd. 54, pp. 304-31Ó. 



1922. «Über den Ursprung der (jrosshirnrinde.» Ana/. An:.. Bd. 55. pp. 33"-3C>5. 



