PETMARY NEUEOMERES AND HEAD SEGMENTATION 339 



after the examination of a large number of specimens decided 

 that they had no definite significance or fate. He regarded the 

 folds in the medullary plate as normal, but not constant, and no 

 evidence was found in the cephalic portion of the plate of divisions 

 to which a segmental value should be assigned. 



Neal ('18) has admirably summarized the evidence for and 

 against the metameric importance of neuromeres. Adducing 

 evidence from a thorough study of the problem of head develop- 

 ment, he contends that the neuromeres of the spinal cord are 

 passive results of the mechanical pressure of the adjacent meso- 

 dermic somites ; that the rhombomeres have arisen in correlation 

 with the visceral arches with which they are functionally con- 

 nected; and that the only structures anterior to the medulla 

 which may h& considered as segmental are the primary forebrain 

 and midbrain segments. 



A number of other investigators have worked on different 

 phases of the head-segmentation problem and a more extended 

 review of the literature may be found in the bibliographies of the 

 papers cited here. 



It is apparent from the disagreement in the results of former 

 investigators that the nature, number, and significance of the 

 neuromeres are far from determined. While neuromeres have 

 been frequently observed in many animals, and widely dis- 

 cussed, the conception of their value as segmental criteria has 

 been largely developed by Locy, Hill, Johnston, and Griggs. 

 It is extremely difficulb, if not impossible, to correlate the obser- 

 vations and interpretations of the various authors, but the 

 repeated observation as to some kind of division in the neural 

 crests and open neural plate is sufS.cient to warrant further in- 

 vestigation. At the suggestion of Prof. J. S. Kingsley, the writer 

 has studied the early stages in Amblystoma and the chick. The 

 work was begun in 1914 and carried on for two years in the 

 zoological laboratory of the University of Illinois. It was 

 interrupted for two years because of military service, but was 

 continued and completed in the biological laboratory of New 

 York University. The writer wishes here to express to Professor 

 Kingsley his appreciation for the many helpful suggestions 



