EVERSION AND INVERSION OF THE DORSO-LATERAL 

 WALL IN DIFFERENT PARTS OF THE BRAIN. 



BY 



C. U. ARIENS KAPPERS, Amsterdam. 

 With Five Figures. 



In the Anatomische Anzeiger, Bd. xxx, 1907, and in a more 

 extensive way in the Folia Neuro-hiologica, Heft 2, 1908, Theu- 

 NISSEN and I have given a comparative description of the dif- 

 ferent forms of forebrain, as they occur in vertebrates, from which 

 it resulted that in Cyclostomes and in Selachians the upper part 

 of the lateral wall of the forebrain is bent in a medio-dorsal direc- 

 tion, forming a sort of pallium, whereas in Ganoids and Teleosts 

 this same part is bent ventro-laterally, so that the primitive brain- 

 mantle which is inverted in the former is everted in the latter. 

 I further called attention to the fact that this primitive mantle 

 should be called palceo-pallnim, as it is older than the archipal- 

 lium, receiving only secondary olfactory fibers, whereas the 

 archipallium receives tertiary olfactory fibers, and further that 

 in those animals where the palaco-pallium is everted it is always 

 greatly reduced in size compared to the inverted paL-eo-pallium 

 of Cyclostomes and especially of Selachians, and that this reduc- 

 tion of the palaeo-pallium gives rise to the formation of the medial 

 epistriatum^ of Ganoids and Teleosts, which has a vicarious 

 function. For this reason we find either a large and inverted 

 palaeo-pallium and a feebly developed epistriatum or an everted 

 palaeo-pallium and a large epistriatum. 



The ontogenetic development of the prosencephalon in Ganoids 

 (Allis, v. Kufffer) makes it probable tu me that these differences 

 find their origin in the form of the skull in embryos, which in a cer- 

 tain stage of development probably pressed on the brain, so that an 

 extensive growth of the dorso-lateral part of the forebrain wall 

 was made impossible. The ventral part of it took then a great 

 deal of its function by means of an enlargement of the striatum 



1 Not visible in Fig. 2, which is drawn after a section anterior to the epistriatum. 



