neal: nervous system in squalus acanthias. 177 



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 ai'e 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 coi'respond 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 Ceiteria of jSTeuromeres. 



I believe that those who find neuromeres in the brain region anterior 

 to the hindbrain have assumed the presence of a horaodynamous seg- 

 mentation of the entire encephalou. 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 vertebrse, 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 nerves. 

 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 metamei-e, 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- 



TOL. XXXI. — NO. 7. 3 



