PEIxM.AJlY NEUROMERES AND HEAD SEGMENTATION 351 



As the neural crests approach each other, the primary bram 

 vesicles and neuromeres of the hindbrain are well defined, and 

 fusion of the folds first occurs in the region of the midbrain 

 vesicle or slightly posterior to it. After the closure of the 

 neural tube there are clearly six segments anterior to the 

 auditory invagination. These brain vesicles and neuromeres of 

 the hindbrain are so well-known that further description is un- 

 necessary. In these divisions there is present the definite, char- 

 acteristic cell arrangement designated by Orr as distinguishing 

 true neuromeres. In the open neural groove there is no arrange- 

 ment of cells in the medullary ridges or floor that even suggests a 

 segmental condition. The cells are evenly distributed and do 

 not manifest any tendency toward groupings that would give 

 morphological significance to the grooves which serve as the 

 basis of the primary neuromerism of Locy and Hill. 



DISCUSSION 



A survey of the literature on the subject of head segmentation 

 shows most unusual differences, both in observation and inter- 

 pretation. These observations are based on the study of differ- 

 ent morphological features, and it has been impossible satis- 

 factorily to explain the discrepancies and differences reported. 

 In the study of neuromerism in urodeles, Kupffer, Froriep, 

 Eyclesh^mer, Neal, Locy, Griggs, Smith, and others have de- 

 scribed as many as eleven and as few as three segments in the 

 cephalic region. Locy considered the divisions of the neural 

 crests as segmental, and did not regard the divisions of the 

 cephalic plate as of metameric importance. Kupffer, Froriep, 

 Eycleshymer, Griggs, Smith, and others have agreed that in 

 any consideration of neuromeric segmentation, the divisions of 

 the medullary plate are of primary importance, but these authors 

 have not agreed as to their number or metameric significance, 

 and most regard them as due to the segmentation of the meso- 

 derm. Locy pointed out that the median divisions do not 

 correspond with those in the neural ridges; he reports four or 

 five divisions in the median plate and ten or eleven segments 

 in the neural ridges of the same region. He says, page 530, 



