334 HORACE W. STUNKARD 



as the results of unequal growth along the margin of the neural 

 plate, he contended that ''it is obviously not necessary to regard 

 such irregularities of the edge of a rapidly expanding plate of 

 tissue as of morphological importance. A disassociation of cells 

 or rapid proliferation of cells, which certainl}^ does occur in this 

 region, would lead to such phenomena." Neal found it im- 

 possible to trace definite segments into the later stages, for in 

 these stages, before the closure of the neural tube, in the majority 

 of specimens little or no evidence of segmentation along the 

 cephalic plate could be seen. In Squalus acanthias he found the 

 posterior boundary of the cephalic plate coincides with the pos- 

 terior boundary of encephalomere VI, opposite which the auditory 

 invagination takes place. Showing discrepancies in Locy's 

 statements regarding the position of the auditory vesicle and the 

 posterior limit of the cephalic plate, Neal says, ^'I can see no 

 escape from the conclusion that he (Locy) has not traced neural 

 segments accurately up to the time they form neuromeres." 

 Furthermore, Neal warned against formulating conclusions 

 from observation of a single organ system and applying them to 

 the phylogenesis of the vertebrate head. He contended that 

 primitively there existed a correspondence between neuromerism, 

 mesomerism and branchiomerism, and the problem of phyletic 

 cephalogenesis is to explain the present lack of correspondence. 

 In Squalus he found five mesomeres alternating with six neuro- 

 meres in the otic and preotic region. 



Hill (1900), working on Salmo and chick embryos, confirmed 

 the statements of Locy. He reported complete agreement as 

 regards the number and position of the neural segments in the 

 trout and chick embryos. The forebrain has three and the 

 midbrain two segments which, in the earhest stages, do not 

 differ in any essential features from those of the medulla. They 

 antedate the historic divisions, forebrain and midbrain, and 

 precede the optic evaginations. The primary neuromeres were 

 constantly and normally present in the early stages of all the 

 embryos examined by him. Speaking of the external and corre- 

 sponding internal constrictions which separate the segments, he 

 says that in the early stages these grooves encircle the encephalon, 



