52 F. W. THYN.G 



except for slight dorso-lateral depressions on either side indicating 

 the area of division into anterior and posterior colUculi. 



Isthmus. The isthmus (Isth.) usually is described as the con- 

 stricted portion between the mesencephalon and the metenceph- 

 alon. It was considered by His ('92), and given in the Basel 

 nomenclature ('95) as a distinct segment of the rhombencephalon 

 which formed a marked ring somewhat narrower dorsally than 

 ventrally. 



In the corresponding area of the brain of this embryo there is 

 on the internal surface of each ventro-lateral wall, a distinct 

 transverse groove. The two grooves unite ventrally in a recess 

 or sulcus (Sul.) caused by a depression of the floor-plate, per- 

 ceptible on the external surface as a distinct elevation (E.i-p.) 

 the eminentia interpenduncularis of His ('92). This sulcus was 

 noted by Burckhardt ('91) and considered by him to be of general 

 occurrence. It was observed in the human embryos by His 

 ('92), and named by him the isthmus groove. Kupffer ('03-'05) 

 named it the sulcus intraencephalicus posterior. The author 

 believes that the transverse grooves mentioned above are the 

 ventral continuations of the caudal mesencephalic neuromere; 

 also that the adjacent brain wall should be regarded as the caudal 

 part of the mesencephalon rather than a distinct division of the 

 rhombencephalon. 



Metencephalon. The metencephalon {Meten.) is the division 

 of the primitive rhombencephalon from which the pons and 

 cerebellum are developed. Its separation from the myelenceph- 

 alon (Myelen.) is indicated, in part, by an internal transverse 

 ridge {Pl.ch.p.) and a corresponding external groove, representing 

 the developing plica chorioidea posterior. 



The roof-plate is enormously expanded so that it forms not 

 only the roof, but the greater part of the lateral wall of the meten- 

 cephalon. The remaining or cephalic part of the lateral wall is 

 formed on either side by a thickened band which is joined cau- 

 dally to the ependymal roof-plate by a thinner intermediate layer 

 known as the rhombic lip. This band on either side represents 

 the corresponding dorsal zone (Z.dors.). It extends obliquely 

 cephalad and medially to merge into the slightly thickened roof- 



