PRIMITIVE SEGMENTATION OF THE VERTEBRATE BRAIN. 459 
sharp line of demarcation, along which the cells of both are 
much crowded together : 
(5) That from the crest of each neuromere, i. e. the external 
convex surface, arises a mass of cells constituting the roots of 
the cranial nerves proper to that region. From the first the 
fifth, from the second the sixth, and from the third the seventh 
and eighth. Opposite to the fourth appears the auditory vesicle. 
From the fifth neuromere the ninth, and from the sixth 
(which he observed not so clearly as the others) the tenth 
nerve arises. Orr observed no neuromeres in the mid-brain, 
but described it as an unsegmented sweep of brain wall which 
was nearly equal in extent to three of the hind-brain neuro- 
meres. In the fore-brain he has described two, similar to 
those in the medulla, save that they give off no nerves. 
Behind the origin of the tenth nerve he found no con- 
strictions. 
I have given these points at length because they formed the 
incentive and the basis of the work immediately afterward 
taken up in this direction, and because they resulted from the 
first definite and systematic investigation of these extra- 
ordinary appearances. 
In 1888 McClure (19) undertook the investigation of these 
neuromeres with a view to demonstrating the neuromeric seg- 
mentation of the neural tube throughout its whole extent. 
By sections of early stages of Amphibian, Reptilian, and Avian 
forms, he corroborated the observations of Orr, except for the 
origin of the abducens nerve, and added the following points: 
(1) That the fore-brain neuromeres conform in every detail 
of structure with a typical hind brain neuromere : 
(2) That the lateral walls of the spinal cord are divided into 
segments, which, while less distinct, are histogenetically similar 
to those of the medulla, and, in fact, are continuous with them 
and the transition gradual. Thus in both regions they must 
have held similar relations to the mesoblastic somites : 
(3) That from the dorsal surface of the segments of the 
spinal cord, or “ myelomeres,” as McClure has named them, 
the roots of the spinal nerves take their origin in the same 
