PRIMARY NEUROMERES AND HEAD SEGMENTATION 347 



Occasional faint lines could be distinguished, but their position 

 and appearance were so irregular and variable that no seg- 

 mental importance could be attached to them. 



Segmentation of the neural folds was reported by Hill ('00), 

 who described the marginal segments as separated by constric- 

 tions that ''in early stages completely encircle the encephalon, 

 and in later stages are confined to its base and lateral walls." 

 In the examination of large numbers of embryos, I have found on 

 the external surface of the neural folds faint constrictions that 

 appear as lines when the best shadow effects are obtained, but I 

 fail to find the regularity described and figured by -Hill. On 

 the contrary, the grooves are irregular in number and position, 

 and often a single groove will divide to form two. The grooves do 

 not regularly encircle the encephalon, but a groove will often 

 fade out at some point and slightly anterior or posterior to it 

 another groove will appear, so that the number of constrictions 

 is different for the two sides. These constrictions are so faint 

 that they can be traced only with difficulty, and in no specimen 

 approxunate the condition shown in Hill's figures. Further- 

 more, dissection shows that internal grooves do not regularly 

 correspond with external constrictions. I find external grooves 

 are present at these stages with no corresponding internal con- 

 strictions and internal grooves with no corresponding external 

 constrictions. Later in ontogenj^ Hill says the internal grooves 

 are elevated upon the apices of internal ridges, but the groove at 

 the apex of the internal ridge is not present with sufficient con- 

 stancy to be of value in determining the constrictions that are 

 of segmental importance. That there are grooves in addition 

 to those considered by Hill to be segmental, he admits when he 

 says, page 423, ''secondary divisions that frequently are present 

 would eventually be confused with the primary ones." But he 

 gives ho criteria by which to distinguish between primary and 

 secondary constrictions, and in his figures certain ones are ex- 

 aggerated as 'primary,' while others are suppressed as 'secondary.' 

 He states that in these early stages the only criteria by which he 

 determined segments are the external and corresponding internal 

 grooves, and that he considers the internal ridge as a secondary 



