PRIMARY NEUROMERES AND HEAD SEGMENTATION 333 



as soon as the neural folds are established, and before there is any 

 division of the mesoderm into protovertebrae. In the open 

 neural groove, the neuromeres of the hindbrain are, he stated, 

 merely the more apparent constrictions of a neuromerism that 

 involves the entire neural plate. He traced the neuromeres to 

 the anterior end of the medullar}' groove and those earliest 

 formed without a break into the later stages, identifying them 

 with the neuromeres of the closed neural tube. In the chick he 

 described neuromeres visible in the blastoderm of the twelfth 

 hour of incubation, and stated that this segmentation extends into 

 the primitive streak. In Aiiiblystoma, at the stage with a 

 broadly expanded neural plate and widely open neural groove, 

 he found "the neural folds divided throughout their length into 

 a series of segments with no especial distinguishing features 

 between those of the head and those of the body region. The 

 median plate included between the neural ridges is smooth at 

 this stage : at a shghtly later period, however, while the groove is 

 still v.'idely open, the median plate exhibits very faint trans- 

 verse markings." He pointed out that these median di\dsions 

 do not correspond with those in the neural ridges, and he attached 

 no morphological significance to them. He claimed that in all the 

 forms studied "The cells in the neural segments are character- 

 istically arranged, even in the earliest stages, and their arrange- 

 ment and structure would indicate that they are definite differen- 

 tiations of cell areas, not merely mechanical undulations." 

 Locy summarized his work on neuromeres by stating that they 

 cannot be artifacts, that they arise before there is any segmental 

 division of the mesoderm, and so cannot be dependent upon the 

 latter. He concluded that neuromeric segmentation is more 

 primitive than mesodermic segmentation, and for this reason 

 msiy well serve as a basis for the study of the segmentation of the 

 head. 



Neal ('98) was unable to verify Locy's statements in Squalus. 

 He found the edges of the plate slightly and irregularly lobed, 

 but the lobes on the opposite margins of the plate did not corre- 

 spond either in number or position, nor did they show any definite 

 relation to the mesodermal somites. Regarding these 'segments' 



