116 B. F. KINGSBUliY 



It was clearly the interpretation of His that the floor-plate ex- 

 tended throughout the neural tube and terminated at the 'an- 

 terior neuropore' where upon the completion of the closure of 

 the neural tube it was continuous with the comparable structure 

 in the roof. His nowhere, however, as far as I can ascertain, 

 devotes to the question any discussion or gives any full state- 

 ment. Later descriptions of the development of the neural 

 tube, e.g., particularly the excellent descriptions of Streeter 

 ('11), conform to this interpretation. Thus in the midbrain the 

 existence of a floor-plate is rather tacitly assumed, as requiring 

 indeed no comment, while in the diencephalon the floor-plate is 

 described. If the histologic characteristics of the differentiated 

 floor-plate as they are known in the region of the spinal cord and 

 rhombencephalon are taken into consideration in our conception 

 of a floor-plate, it becomes apparent that the floor-plate has a 

 much more limited extent than is at present the interpretation. 

 In other words, if we define the floor-plate as the medial ventral 

 portion of the neural tube, consisting of neuroglia (ependyma) 

 alone, devoid of neuroblasts and furnishing therefore no neuronal 

 elements, but separating the nervous system (neural plate) into 

 two primary right and left halves, a floor-plate is lacking, as such, 

 in the mesencephalon and diencephalon. The floor-plate would 

 then be recognized as extending throughout the spinal portion 

 of the neural tube and the rhombencephalon, up to a structural 

 feature of considerable morphologic significance, the fovea isthmi, 

 and there rather abruptlj' terminating. Cephalad of this point, 

 that is, in the floor of the midbrain, we find, not ependyma 

 alone, but differentiation in terms of ependymal, marginal, and 

 mantle zones, such as is encountered in the lateral walls of the 

 neural tube. 



The fovea isthmi, to which attention is drawn when seeking 

 the anterior end of the floor-plate, requires brief notice in pass- 

 ing. It was first described by Stieda (75), Burckhardt ('91) and 

 myself ('95) in the amphibian brain. At that time I referred to 

 it as the mesencephalic pit. His had previously termed it in 

 the embryo the 'Isthmusgrube' ('92, 1), and indicated its pres- 

 ence in frog, shark, salamander, trout, sturgeon, chick, and man. 



