EXTENT OF THE FLOOR-PLATE OF HIS 129 



significance as fundamental segments of the neural tube. The 

 telencephalon and diencephalon would then be entirely developed 

 out of alar-plate material. This is the interpretation of Schulte 

 ^and Tilney ('15). 



7. The conception of the floor-plate as marking a line of blasto- 

 poric concrescence, and representing the neural portion of a 

 neurochordal suture postulates that when first formed the noto- 

 chord and floor-plate be coextensive. It would thus be possible 

 to speak of a prechordal portion of the neural plate as a primitive 

 condition, as well as pre- and epichordal parts of the neural tube. 



Ahlborn ('83), in the lamprey, so divided the brain into epi- 

 chordal and prechordal regions, basing the distinction drawn 

 upon three things: 1) The epichordal brain alone possesses 

 nerves comparable to those of the spinal cord; such are lacking 

 in the prechordal portion. 2) It possesses a raphe which is com- 

 pletely lacking in the prechordal brain. 3) It is coextensive 

 with the notochord. Ahlborn thus included only the rhomben- 

 cephalon in the epichordal brain, the mesencephalon and prosen- 

 cephalon being prechordal. Strong ('16), doubtless through an 

 appreciation of the force of the first of Ahlborn's points, has like- 

 wise divided the brain into epichordal and prechordal parts, ^ 

 but includes the mesencephalon in the epichordal brain, and the 

 reason for this is obvious; to the midbrain belong the oculomotor 

 and trochlearis nerves. The plan of brain morphology here out- 

 lined limits the epichordal brain more nearly to the rhomben- 

 cephalon, essentially as was done by Ahlborn, in the lamprey. 

 The difficulty in recognizing a motor zone cephalad of the chorda 

 is, however, obviated. I also see no reason why the nuclei of the 

 Ilird and IVth cranial nerves might not vary in position in 



* (P. 453.) "After closure (of the neural tube), in manj^ Vertebrates at least, 

 a three-fold division can be made out: (1) A caudal part of the neural tube, the 

 spinal cord, which gradually expands cranially into (2) the caudal part of the 

 brain (deuterencephalon v. Kupffer) (fig. 400). These two parts lie above the 

 notochord and all the typical cerebrospinal nerves are connected with them. 

 (3) Cranially, at the anterior end of the notochord, the brain wall expands ven- 

 trally forming the third portion (archencephalon). At the forward extremity is 

 seen the anterior neuropore. The deuterencephalon is thus an epichordal part 

 of the brain, while the archencephalon is prechordal." 



