PRIMARY NEURO-MERES AND HEAD SEGMENTATION 349 



modification. Observation of a large number of embryos affords 

 no evidence to support the statement of Hill that ''eleven con- 

 strictions are present on both inner and outer surfaces of the 

 open neural groove, that they are constant in number and 

 nearly equal in size and that they appear earher in ontogeny 

 than the historic encephahc divisions, forebrain, midbrain, and 



Fig. 15 Frontal section through the neural crests of an embryo of three 

 somites, showing the irregular lobulations of the crests, absence of corresponding 

 external and internal grooves, and indefinite cell arrangement. 



hindbrain." Hill reported that the third and fifth grooves are 

 deeper than the others and mark the posterior limits of the fore- 

 brain and midbrain. While it is true that with the appearance 

 of the so-called secondary division of the neural tube into fore- 

 brain, midbrain, and hindbrain, the limits that separate them are 

 clearly marked, the present study fails to confirm his statement 



