NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. INT 
encephalomeres differ. While a mechanical explanation is possible for 
the one, such is not possible for the other. They are, it is true, con- 
tinuous serial dilatations of the neural tube. The proof, however, that 
they are of equal morphological value, that is to say, serially homologous, 
rests, I believe, in the demonstration of a similar metameric relation to 
organs known to be segmental. The myelomeres correspond meta- 
merically with the somites, as has been stated. Do the encephalomeres 
likewise correspond with somites? Upon the answer to this question 
obviously depends largely the decision as to their metameric value. Be- 
fore stating the evidence bearing upon this question it is necessary to see 
if there is any evidence of neuromeres anterior to the hindbrain. 
IV. The Neuromeres anterior to the Hindbrain. 
a. ESSENTIAL CRITERIA or NEUROMERES. 
I believe that those who find neuromeres in the brain region anterior 
to the hindbrain have assumed the presence of a homodynamous seg- 
mentation of the entire encephalon. Yet it must be admitted that even 
if a serially homologous segmentation extends from the spinal cord 
into the medulla oblongata, it by no means follows that such segmenta- 
tion also extends into the anterior brain region. Compare with the 
analogous case of the skull. Because the occipital region is segmen- 
tal, i. e. composed of fused vertebra, it does not follow that the pre-otic 
region is. It is well, at least, to study the conditions in the anterior 
brain region with the mind as unprejudiced by any theory as possible. 
What criteria, then, warrant the conclusion that any given division of 
the neural tube is a neuromere ? Certainly, no one criterion would 
be held to be sufficient. The best criteria are such as associate the 
supposed neuromeres metamerically with other structures known to 
be segmental, e.g. the mesodermic somites or the segmental nefves. 
But where such direct evidence is wanting, to say that a radial arrange- 
ment of cells and nuclei is evidence of a neuromere, and thus indirectly 
evidence of a metamere, is obviously dangerous, since the radial arrange- 
ment of the nuclei appears whenever the neural tube is constricted from 
any cause whatever, 
If, however, we have rudimentary somites in the head, may we not 
also have rudimentary neuromeres? McClure (89) finds between the 
midbrain and the optic vesicle of the Lizard a structure which resembles 
a portion of a neuromere, —a “half-neuromere.” He accepts the evi- 
VOL, XXXI. —NO. 7. 3 
