EXTENT OF THE FLOOR-PLATE OF HIS 117 



Corresponding to the medial depression internally, His described 

 a prominence externally to which he gave the name of eminentia 

 interpeduncularis. Herrick ('17) has recently reexamined the 

 fovea isthmi in the amphibian brain. It may conveniently be 

 regarded as marking the caudal boundar}^ of the midbrain on the 

 medial floor. Herrick uses the term fovea isthmi, which is here 

 adopted. Von Kupffer ('06) terms it the 'sulcus intraencephali- 

 cus posterior.' He figures it in all classes of vertebrates save the 

 clyclostomata and the Mammalia (the latter class was not in- 

 cluded in the limits of his presentation). Johnston ('09), who 

 figured the medial plane in the brains of shark, salamander, and 

 mammal, shows the fovea isthmi only in the salamander. In the 

 medial plane reconstructions of the developing brain of shark, 

 bird, and mammal, reproduced here as figures 1, 2, and 3, it is 

 evident in all. The floor of the midbrain anterior to the fovea 

 isthmi is without adequate embryological designation. For the 

 German terms Haube, Haubenwulst as applied to the mesence- 

 phalic floor, 'tegmental swelling,' 'tori tegmentales,' and 'tori 

 semicirculares' have been used. The 'tuberculum posterius' 

 of von Kupffer may be taken as marking the anterior boundary 

 of the mesencephalic floor. 



To illustrate the abrupt termination of the differentiated floor- 

 plate at the fovea isthmi, three reconstructions were made of 

 the midsagittal plane, from three classes of vertebrates, the 

 elasmobranch (Acanthias), the bird (chick), and the mammal 

 (calf). These are reproduced as figures 1, 2, and 3. It will be 

 seen that the differentiated floor-plate, characterized by the pres- 

 ence of the ependymal layer only and the neuroglial processes 

 (neuroglia fibers), often grouped together in parallel radial bun- 

 dles, terminates at the fovea isthmi and cephalad of this point, 

 in the floor of the midbrain, the characteristic arrangement of 

 the floor-plate is lacking, while ependymal, mantle, and marginal 

 layers appear. In other words, the differentiation shown is 

 that characteristic of the lateral wall of the neural tube. Ceph- 

 alad of the midbrain the medial floor thins in the hypothalamic 

 region, to thicken again at the chiasma. Inasmuch as the low 

 magnification for these medial plane reconstructions necessitated 



