434 Leland Griggs. 



ponents and the mesodermal somites of this region have under- 

 gone profound alterations. If, as the most recent investigation 

 seems to show, the nervous system, appearing first, presents 

 a simpler and more unaltered condition than the other two 

 systems, then it may well serve as a basis for the study of seg- 

 mentation of the head; and other organs should be shown to 

 correspond to it rather than vice versa. 



From this survey of the literature dealing with the segmen- 

 tation of the nervous system of the Amphibia and allied forms 

 it seems fair to conclude that the presence of neuromeres has 

 been proved conclusively. As Hill ('00) says, ''There is sub- 

 stantial agreement among observers as to the six segments in 

 the hind-brain." The problem of the segmentation of the 

 fore-brain and mid-brain, however, may still safely be con- 

 sidered unsolved. While the evidence in favor of the segmen- 

 tation of the neural crests and the open plate seems very strong, 

 the difference of opinion as to the constancy, number and rela- 

 tions of the segments is so great that considerable more research 

 will be necessary before we have a fully established theory of 

 primary neuromeres. 



In solving the problem of the morphology of the vertebrate 

 head the subject of sense organs is second in importance to that 

 of neuromeres. The origin of the olfactory organ, of the eyes 

 both lateral and parietal, of the ear and the lateral line organs 

 have all been made the subject of- careful investigation, par- 

 ticularly in the Amphibia and other groups among the lower 

 vertebrates. "For the purposes of this paper only one of these 

 topics will be considered in detail, that of the origin of the lateral 

 eyes. 



It was observed by Balfour ('81) that the anlage of the eye 

 appeared before the closing of the neural canal. ■ He has quoted 

 with approval a statement by Lankester that the ''original 

 vertebrate must have been a transparent animal with an eye or 

 pair of eyes within the brain." 



Whitman ('89) observed the anlage of the eyes on the surface 

 of the amphibian egg in the open neural plate stage. In Nec- 

 turus before the neural crests have begun to move together he 

 found that "the basis for the eye is already discernible as a cir- 



