432 CHARLES HILL, 



The study of cephalic mesoraeres has led to greater divergence 

 of opinions and more conflicting views than is generally supposed. 

 In Elasmobranchs, the only group in which their developmental history 

 has been traced, there is no consensus of opinion as to their origin, 

 number, and morphological values. Van Wijhe, Dohrn, Killian, Miss 

 Platt, hold that they are all fundamental segmental divisions of the 

 cœlomic cavity. Gegenbaur, Sedgwick, Rabl, Sewertzopf maintain 

 that some of them probably represent gill clefts and other divisions 

 of the cœlomic cavity. In the Elasmobranchs, in the pre-otic region, 

 from 3 to 14 segments are recorded (Rabl, '92, and Dohrn, '90). 

 Moreover, some observers conclude that certain cephalic mesomeres 

 have disappeared in phylogenetic history (Sedgwick), while others 

 claim a fusion of segments has taken place, and still others maintain 

 that primary segments have subdivided and in this way have pro- 

 duced a larger number of "head-cavities" than were originally present 

 in the ancestral Vertebrates. 



In sequence of time they are said to develop from before, back- 

 wards. Gegenbaur, '87, insists that we should carefully observe a 

 distinction between paliugenetic (primary) and cœnogenetic (secondary) 

 somites. This is philosophical and supported by fact of development. 

 It has been generally adopted (Kastschenko, '88, Sewertzoff, '95, 

 HoFFiMANN, '97). The segmental divisions of the mesoblast has been 

 shown to begin in the region of the auditory vesicle and develop 

 backwards. This gives us the post-otic or palingenetic segments. The 

 mesoblast in front of the auditory capsule is afterwards divided into 

 segments designated as pre-otic or cœnogenetic. 



The cœnogenetic segments are found to ditier in form, size, histo- 

 logical conditions as well as time of development. Some of these 

 segments are solid, while most of them show a cavity. They may be 

 partially or completely separated from the adjacent ones, united or 

 separated ventrally, and also occupy a lateral or a ventral position 

 with reference to the neural axis. 



Since the study of their developmental history, in the Elasmo- 

 branchs, has led to such a variety of views, their morphological inter- 

 pretations vary in like degree and this weakens their value as seg- 

 mental criteria. Some authors assign to all cephalic mesomeres a 

 fundamental segmental value. Some hold the post-otic only have this 

 value, while the pre-otic are interpreted as mechanical structures or 

 evaginations correlated with the gill clefts. 



The recorded observations on the cephalic mesomeres in other 



