EXTENT OF THE FLOOR-PLATE OF HIS 119 



that the representation of the structure be quite diagrammatic, 

 photographs from the sections showing the median plane at the 

 fovea isthmi are reproduced as figures 7, 8, and 9. Of these, that 

 of the calf embryo shows the median plane throughout, while in 

 figures 8 and 9 only the extent indicated is medial. In figure 9, 

 the pulling away of the neural tube from the mesenchyme has 

 accentuated the 'eminentia,' making it more abrupt, while the 

 outline of the external surface cephalad of it is poorly defined. 

 It will be seen, particularly in the reproductions from the calf 

 and shark embryos (figs. 7 and 8), that the statements made 

 above are fully confirmed by the photographs. 



Two additional photographs from frontal sections of a calf em- 

 bryo (20 mm.), through the summit of the mesenchyme in the 

 plica encephali ventralis^ and somewhat more dorsally are given 

 for comparison (figs. 10 and 11). The general position oi these 

 levels is indicated upon the medial plane reconstruction (fig. 3) 

 by the lines *a' and 'b.' It may be noted that in figure 10 

 the floor-plate is shown in the caudal limb of the bend and its 

 absence in the cephalic (mesencephalic) limb or portion. The 

 more dorsal section (fig. 11) is through the cephalic end of the 

 floor-plate where the letter 'c' indicates its limit. 



The question of the significance attaching to the termination 

 of the floor-plate anteriorly at the fovea isthmi involves the in- 

 terpretation of the cephalic portion of the neural tube, and 

 brings up for consideration the much-discussed question of the 

 anterior end of the neural plate and of the neural tube itself. 

 The conclusions reached by His were given in 1888 and particu- 

 larly in 1892 in his paper on the general morphology of the brain. 

 They are well illustrated by his figure 1 from the latter article, 

 which is reproduced here as figure 4. The anterior end of the floor 

 of the neural tube he placed at the 'Basilarleiste' 'B' (basilar 

 fold) later to become, accordng to his interpretation, the recessus 

 infundibuli. The dorsal wall of the tube, due to its great curva- 



3 This term introduced by v. Kupffer and meaning, as its composition implies, 

 'ventral brain fold,' seems to the writer more useful in this connection than 'ceph- 

 alic flexure' ('head fold') for which it is a partial equivalent. 



