620 



COMPARATIVE ANATOMY 



nervous system. In annelids and arthropods each metamere typically 

 contains a well-marked ganglion which serves as a nerve center for the 

 segment. In some arthropods in which the metameres have been tele- 

 scoped together, the original segmentation is still represented in a series 

 of metamerically arranged gangUa. Here the neuromeric segmentation 

 persists when other signs of metamerism are all lost. The fact of the 

 transiency of vertebrate neuromeres has not counted against their value 

 as criteria of segmentation, but in their favor, as vanishing remnants of the 

 metamerism of invertebrate ancestors. 



A.AMPHIOXUS 



C. SQUALUS. 



B.PETROMYZON D. CALLUS 



Fig. 514. — Diagrams of the neuromeres in chordate embryos. Wanting in Amphioxus 

 (A), they appear with increasing distinctness in Petromyzon (B), Squalus (C), and 

 Chick (D). This evidence does not accord with the assumption that they are pre- 

 chordate structures which appear transiently in vertebrate embryos. 



Compared, however, with the results based upon mesomeres (somites), 

 the study of neuromeres has proved disappointing. Students of neuro- 

 meric segmentation agree as to the existence of hindbrain neuromeres 

 or rhombomeres, but as to little else. The discrepancies in their results, 

 combined with the increasing distrust of the theory of the annehd ancestry 

 of vertebrates, have served to weaken confidence in conclusions based 

 upon the study of neuromeres. Moreover, most morphologists are 

 convinced that metamerism began in the muscular, not in the nervous 

 system. Nervous metamerism is, in their judgment, a secondary adapta- 

 tion to muscular segmentation. 



Locy ('94) and his pupil Hill ('00) claim to have found a uniformly 

 beaded segmentation throughout the length of the brain and spinal cord. 

 Their highly diagrammatic figures have found their way into recent 

 textbooks, in spite of the fact that the majority of investigators have 

 failed to confirm their results. As a matter of fact, the serial homology 



