120 CHARLES EUGENE JOHNSON 



divided into a number of segments, called by him head cavities. 

 These head cavities, later looked upon as true somites, became, 

 as is well known, of considerable importance in connection with 

 the question of the original segmentation of the vertebrate head. 



The bulk of the work on the head cavities has been done on 

 fishes, , especially elasmobranch embryos, and a varying number 

 of such cavities has been found in different members of the group, 

 nine being probably the average. The head cavities or somites 

 lying in front of the otic region have furnished the greater inter- 

 est, as here evidence of segmentation has been effaced to a greater 

 degree than in the opisthotic region, where cranial nerves of the 

 vagus group, well differentiated somites, and the arterial and 

 gill arches have been considered by investigators as strong evi- 

 dence of metamerism. The scarcity of metameric evidence in 

 the prootic region is associated with the progressive develop- 

 ment of the brain and associated sense organs, so that the higher 

 we proceed in the vertebrate series the more obscure become the 

 traces of any metamerism which may have existed in the pre- 

 cursors of the group. 



The problem of the head somites in the Amphibia has received 

 little attention, although Scott and Osborn as early as 1879 

 found that a portion of the coelom was present in the head and 

 became segmented by the development of gill clefts; and Miss 

 Piatt ('94) found soitiites in Necturus corresponding to the pro- 

 otic somites of elasmobranchs. In the Mammalia no head cavi- 

 ties can be said to have been found, although Zimmermann ('99), 

 for a human embryo of 3.4 mm., has described a number of small 

 but clear-cut vesicles which may have been vestiges of such 

 structures. For reptiles and birds on the other hand, cavities 

 or somites correspondiD.g to the first three head somites of elasmo- 

 branchs have been established. These cavities or somites accord 

 with the prootic somites of the latter group in that they occupy 

 corresponding positions in the mesoderm, have corresponding 

 nerve relations, and in each case give rise to corresponding mus- 

 cles of the eye-ball. But whether a particular somite in one 

 class of vertebrates is strictly homologous with a particular somite 

 in another group, is of course uncertain. 



