374 J. B. JOHNSTON 



rise to something more than the hippocampal formation in the 

 mammalian brain (Johnston '10 c, p. 149). With the study of 

 the brains of a number of reptiles and mammals, the conviction 

 has gradually forced itself on the writer that the prevailing ideas 

 regarding the morphology of the medial wall of the telenceph- 

 alon in mammals must be modified in view of the fundamental 

 relations in lower vertebrates. The evidence for this can best 

 be presented by careful comparison of the chief structures in 

 this region in lower and higher vertebrates. 



Eor the sake of clearness it must be understood that the term 

 primordium hippocampi is used in the sense in which the writer 

 has employed it in his previous papers. The fitness of this and 

 other terms is discussed in a section at the end of this paper. 



THE ROOF-PLATE IN THE TELENCEPHALON 



The anterior end of the brain floor is occupied by the optic 

 chiasma ('09 b). The roof-plate begins at the preoptic recess 

 and extends in a curve around the topographic rostral end of 

 the brain and caudad into the roof of the diencephalon ('10 c). 

 The velum transversum, always present in vertebrates at least 

 in embryos, marks the boundary between diencephalon and 

 telencephalon in the roof ('09 b). The telencephalic roof-plate 

 consists of three portions, the lamina terminalis, lamina supra- 

 neuroporica and tela chorioidea ('11 b, p. 491). The lamina 

 terminalis is formed by the fusion of the lips of the primitive 

 neuropore and is bounded above by the neuroporic recess which 

 marks the point of latest connection of the neural tube with the 

 ectoderm. The lamina supraneuroporica is thickened by com- 

 missural fibers in cyclostomes and by gray matter and fibers in 

 selachians, and even in ganoids and bony fishes where it usually 

 contains no commissures it is thickened and is histologically 

 different from the tela chorioidea ('lib). From the tela chorioi- 

 dea just rostral to the velum transversum arises the paraphysis. 

 The attachment of the tela chorioidea to the dorsal border of 

 the wall of the telencephalon medium is known as the taenia 

 fornicis ('11 a, p. 6). The taeniae of the two sides converge ros- 

 trally to meet at the point of junction of the tela chorioidea 



