Developmental history of primary segments of the vertebrate head. 435 



been found only in a few Vertebrates (Elasmobranchs, Amphibia and 

 Reptiles), and in Elasmobranchs, the only group in which their develop- 

 mental history has been described, the authors who have given the 

 subject the most critical study report conflicting and contradictory 

 views. On the other hand, neural segments of acknowledged morpho- 

 logical value have been observed in the medulla by many'authors in all 

 the vertebrate groups. These segments can be traced to very early stages 

 before the fore-, mid- and hind-brains have appeared, or mesomeres 

 have developed. In these early stages the neural segments are 

 constant in number and identical in form, but owing to the rapid 

 changes that take place in the anterior portion of the neural axis, 

 the anterior encephalic segments disappear before those of the medulla. 



Summary. 



In Teleost an chick embryos the encephalic portion of the neural 

 axis is divided into similar joints or segments which in early stages 

 are present in the region of fore- and mid-brain — - secondary modi- 

 fication in the anterior encephalic region of the chick soon obliterates 

 all traces of primitive segments, but the original joints persist for 

 some time in the medulla which is less modified. In the Teleosts 

 the segments in front of the medulla are less transient than those of 

 the chick. 



In the medulla the original joints persist for some time. Secon- 

 dary expansions of the mid-brain and fore-brain, arising after the 

 primary joints of this region fade away, have been mistaken for 

 primary segmental divisions, and made co-ordinate with the persisting 

 primary segments of the medulla which are in realty identical with 

 the first formed segments in the anterior brain region. 



The three anterior segments represent the region of the fore- 

 brain, the next two the region of the mid- brain. These five segments 

 differ in no essential feature from the segments of the medulla. 



The sixth segment forms the cerebellum, and the seventh to 

 eleventh, inclusive, represent the medulla, making a total of eleven 

 encephalic segments. These segments are constantly and normally 

 present in the early stages of all the embryos examined. 



