THE HEAD PROBLEM 



621 



of neural divisions anterior and posterior to the neuromeres of the medulla 

 has not been demonstrated. 



A number of facts appear to invalidate the assumption that neuro- 

 meres are reliable criteria of the primitive segmentation of the head. 

 Neuromeres are more conspicuous in the embryos of higher than in those 

 of lower vertebrates, and it was evidence similar to this that led to the 

 abandonment of the vertebral theory of the skull. Neuromeres in cyclo- 

 stomes appear to be limited to the medulla and even there are identified 



"ST yn 



' "Orr^NOTOCHORD. 



i— NOSE 

 MOUTH PLACODE—^ K eopoQc'^ 

 Fig. 515. — A median section of a young Petromyzon embryo, showing the neuro- 

 meres. The neuromeric segmentation of Petromyzon appears to correspond with the 

 primary neuromeric segmentation of elasmobranchs and birds. It is, however, less 

 marked in Petromyzon than in these higher forms. (Redrawn after KoltzofE.) 



with difl&culty; but they become increasingly prominent as we pass in 

 the vertebrate series from fishes to mammals. Such evidence does 

 not accord mth the assumption that neuromeres have phylogenetic 

 significance. 



As stated above, little resemblance is seen between rhombomeres 

 (neuromeres of the medulla oblongata) and the neural segments anterior 

 and posterior to them. Since rhombomeres appear chiefly in the lateral 

 plates of the medulla, the only structures comparable anteriorly are the 

 forebrain and midbrain vesicles, not secondary subdivisions of these. 

 The so-called neuromeres in the trunk region are transient effects produced 

 by the pressure of the mesodermal somites in early stages of their develop- 

 ment, and affect only the ventro-lateral portions of the spinal cord. Such 

 evidence leads to the conclusion that a rhombomere is a new structure, 

 possibly developed in relation to the visceral arches and clefts. 



Furthermore, the cranial nerves do not have simple one-to-one rela- 

 tions with neuromeres, as might be expected if they are metameric 



