Developmental history of primary seffments of the vertebrate head. 433 



Vertebrates, are so limited that we have little knowledge regarding 

 their presence or possible conditions. The controversies, therefore, 

 are almost exclusively confined to the observations that have been 

 recorded upon the mesoraeres of Elasmobranch embryos. 



While it must be admitted that the cephalic mesoderm may carry 

 the evidence of a primitive metamerism, it seems to me that all the 

 observations go to show that there is too little uniformity about the 

 divisions of this tissue to make it a favorable structure for deter- 

 mining the number and relationship of cephalic segments. 



b) Branchiomeres. The weakness of branchiomeres as clues 

 to the original segmentation of the head has been well stated by 

 MiNOT, '92: "The relation of the nerves to the segments (myotomes 

 and neuromeres) are primitive ; the relations to the branchial arches 

 and gill-clefts are secondary. Indeed we must assume that the Verte- 

 brates had segmental ancestors, who acquired gill-clefts, segments 

 being phylogenetically much older than gill-clefts. The ancestral 

 nerves were adapted to gill- clefts and we may some day know the 

 history of that adaptation and the modifications consequent upon it. 

 At present we can only say that, contrary to the assumption which 

 has prevailed for twenty years, the gill-clefts are not segmental and 

 therefore the branchial nerves are not in segmental order." (Human 

 Embryology, p. 636.) 



c) Neural Segments. The case seems to stand somewhat 

 better for the neural segments. Those of the medulla are accepted 

 by most morphologists as representing true joints or segments. It is 

 those of the fore- and mid-brain regions whose presence and segmental 

 value is questioned. The identity of those of the medulla and their 

 relation to nerves is well established in all the Vertebrate groups 

 from fishes to mammals, unfortunately, but few observers have at- 

 tempted to trace their complete history. The observations have been 

 confined almost exclusively to late stages and the neural segments, 

 therefore, have come to be looked upon as structures that follow 

 rather than precede the segmental divisions of the mesoblast. Loct 

 was the first to observe the early stages of the neural segments. He 

 found that they could be traced back to stages that antedate the 

 formation of mesomeres. The significant fact is that Locy traced them, 

 as his sketches show, from their earliest appearance through the 

 stages with an open, a closing, and a closed neural groove, and 

 identified those of the medulla with the neuromeres of other authors. 



Neal, '98, found segmental divisions in early stages of sharks 



