420 C. JUDSON HERRICK 



They conform in morphological type to those figured by Sterzi 

 in Acanthias ('09, p. 577, fig. 232). 



Sections of much younger Amblystoma larvae of 10 mm. (about 

 15 days) so oriented as to cut the brain horizontally in the region 

 of the velum trans versum (figs. 6 and 7) illustrate clearly the 

 relations of the velum (r) to the eminentia thalami and adjacent 

 parts and the communication of the diencephalic and telenceph- 

 alic ventricles through the wide aula in front of the velum. 



The only massive connection between the hemispheres and the 

 diencephalon at this age is by way of the eminentia thalami or 

 structures lying farther ventrally which connect the ventral 

 parts of the hemisphere with the ventral nuclei of the thalamus 

 below the sulcus medius. The dorsal parts of the hemisphere 

 are not well differentiated at this age, but comparison with older 

 stages of Amblystoma and other Amphibia furnishes abundant 

 confirmation of the statement last made. 



The sections figured are so inclined as to show clearly that the 

 connection between the dorsal part of the posterior pole and the 

 wall of the thalamus is at this age wholly membranous (figs. 6 

 and 7, z). That is, it is comparable with the posterior chorioidal 

 fold which is so conspicuous a feature of the brains of all embryonic 

 reptiles and mammals. The fact that mitotic figures are more 

 abundant around this angle of the lateral ventricle than elsewhere 

 suggests that this is the point of most rapid growth at this age. 

 In immediately following stages this fold is all incorporated into 

 the extensive lateral plexus whose earliest rudiment is seen in fig. 

 6 at y (cf. fig. 3) and the massive tissue of the posterior pole rests 

 in immediate contact with the eminentia thalami (figs. 3-5). 

 There is at no stage in any of the Amphibia which I have exam- 

 ined a direct massive connection between the dorsal parts of the 

 cerebral hemisphere and the pars dorsalis thalami (dorsally of the 

 sulcus medius) or the epithalamus. Here, as in Anniota, the im- 

 portant fibrous connections between these dorsal parts of the 

 telencephalon and diencephalon all cross the dorsal barrier inter- 

 posed by the velum transversum and di-telencephalic fissure in 

 the massive ventral parts. See p. 430 and the discussion on pp. 

 474 and 486. 



