EXTENT OF THE FLOOR-PLATE OF HIS 123 



the floor of the brain cephalad of this point would thus represent 

 a secondary concrescence of the edge of the neural plate. In 

 other words, in terms of the His nomenclature, they would extend 

 the frontal suture caudally in the floor so as to include in its ter- 

 ritory not only the optic chiasma (as did His) , but the infundibu- 

 lum as well. Their interpretation thus departs widely from that 

 of Johnston. They approach his interpretation, however, in 

 deriving the infundibular recess from the primitive optic vesicles.^ 



No comment need be made on the interesting conchisions of 

 Schulte and Tihie}^ save to emphasize that the critical point is 

 the interpretation of the 'tubercle of the floor' and to venture the 

 opinion that the figures offered in illustration neither conclusively 

 show that it corresponds to the anterior end of the neural plate 

 nor that it marks the anterior boundary of the mammillary re- 

 cess. The floor plate, as such, and the question of its extent, they 

 do not specifically discuss. 



As is of course well known. His had in a number of articles 

 ('76, '77, '77, '91) proposed and supported the view that the 

 axis of the vertebrate body was established by the concrescence 

 of a 'germ ring.' His last discussion of concrescence ('91) was a 

 paper read at the meeting of the Anatomische Gesellschaft, and 

 in the discussion his conception of concrescence met with con- 

 siderable adverse criticism. He therefore welcomed the support 

 0. Hertwig ('92) gave to the theory of concrescence through the 

 publication the next year of his classical paper, ''Urmund und 

 Spina bifida." Hertwig's conclusion, however, that the 'neuro- 

 chordal seam' was laid down by the concrescence of the right and 

 left halves of the (dorsal) blastoporic lip, he could not accept. 



^ Schulte and Tilney ('15), (p. 3-40): "As the tubercle of the floor constitutes 

 the extremity of the floor-plate and at the same time the primitive ventral lip 

 of the neuropore, it is of prime importance to ascertain its position in subsequent 

 stages of development." (P. 34L) "The tubercle of the floor is now losing its 

 demarcation from the parietes with the effacement of the primitive ventral seg- 

 ment of the optic sulcus, and from this period appears as a transverse ridge inter- 

 vening between the mammillary and infundibular regions. It is, therefore, 

 evident that the mammillary region arises from the "ephalic extremity of the 

 primitive floor-plate and that the infundibular region is a derivative of the 

 primitive optic vesicles." 



THE JOURNAL OF COMPARATIVE NEUROLOGT, VOL. 32, NO. 1 



