460 Journal of Comparative Neurology and Psychology. 



the recessus prceopticus. In the walls of the lateral ventricles are 

 found the secondary olfactory centers and the corpora striata. To 

 these in higher forms are added more complex cortical structures 

 which in mammals become the predominant part of the telencephalon. 

 The above mentioned general features of the forebrain are univer- 

 sally recognized and require no further comment. Other features 

 remain less well understood or their interpretation is in dispute so 

 that they present problems for solution. These problems may be 

 stated here, and will be discussed after the description of certain 

 features in the embryonic and adult brain which contribute to their 

 solution. 



1. The anterior end of the brain. It is generally recognized that 

 the lamina terminalis represents the seam of closure of the anterior 

 neuropore, but the location in the adult brain of the lower border of 

 the neuropore is placed by different authors at various points between 

 the anterior commissure and the infundibulum. The determination 

 of the anterior end of the brain is of fundamental importance for 

 fixing the segmental order of the parts of the forebrain along the 

 brain axis. Thus, if the anterior end of the brain is at the infundi- 

 bulum, the optic portion of the hypothalamus . together with all the 

 telencephalon would be dorsal structures, while if the anterior end 

 lies at the preoptic recess the optic part of the hypothalamus must 

 be considered as belonging to the ventral portion of the brain. In 

 the latter case the olfactory centers would belong to the first segment, 

 in the former they would fall behind the optic chiasma around the 

 dorsal margin. 



2. The boundary between the diencephalon and telencephalon. 

 This problem is closely bound up with the first one. Although His 

 placed the anterior end of the base of the adult brain behind the 

 infundibulum and included the pars optica hypothalami in the telen- 

 cephalon, most authors have referred the pars optica to the dien- 

 cephalon and this is the obvious meaning of the tables of the BISTA. 

 In the roof of the brain of true fishes and amphibians and in the 

 brains of embryos of most vertebrates occurs a well marked trans- 

 verse fold of the membraneous roof, the velum transversum.. This is 

 considered as marking the boundary between di- and telencephalon in 



