PEIMARY NEUROMERES AND HEAD SEGMENTATTON 337 



headings or lobes on the neural crests, but did not regard them 

 as of morphological importance. These lateral lobulations he 

 found vary both in number and arrangement and as the neural 

 crests close over the plate all signs of segmentation behind the 

 procephalic lobes disappear. He described three distinct grooves, 

 the anterior and posterior germinal depressions and the 'blasto- 

 groove' which appear in the location later occupied by the 

 neural groove. Griggs stated that the primary neuromeres 

 described by Loc}^ were not apparent in any of the embryos 

 which he examined. He concluded that the median transverse 

 grooves separate the true neuromeres, that the first contributes 

 to the formation of the forebrain, the second and third to the 

 formation of the midbrain, and the fourth to the formation of the 

 anterior part of the cerebellum. 



Smith ('12) described grooves which appeared very early in 

 the neural place of Cryptobranchus embryos, one regularly 

 antedated the others, and this I believe corresponds to the 

 transverse cephalic groove of Griggs. Anterior to this groove he 

 noted six transitory furrows and posterior to it an undetermined 

 number, but expressed the suspicion that these grooves are 

 connected with the formation of the mesodermal somites. In 

 early stages of the formation of the neural folds he observed 

 occasional transverse grooves, but stated that they are often 

 irregular and bear no definite relation to the segments of the neu- 

 ral plate. He argued that the true segments are to be found 

 between the transverse grooves of the neural plate, and pointed 

 out that in the region of the mesodermal somites the transverse 

 grooves of the plate are in line with the intersomitic grooves and 

 the neuromeres are in line with the somites. He was unable to 

 follow the various structures of the neural plate into the defini- 

 tive divisions of the einbryonic and adult brain. 



Graper ('13) reviewed the literature on neuromeres exten- 

 sively; he criticised Hill's ''iiberraschenden und von niemand 

 bestatigten Zeichnungen" and was unable to confirm his obser- 

 vations in the chick. 



Neal ('14) argued that the hindbrain neuromeres manifest a 

 segmentation that cannot be explained upon purely mechanical 



