ORIGIN AND STRUCTURE OF THE HEAD 79 



head in Elasmobranchs disappears soon afterwards and the 

 mesoderm segments are dissolved and replaced by an 

 unsegmented mass of mesenchyme, as is observed in this 

 region from the beginning in Amniotes. This mesenchyme, 

 however, according to FRORIEP, is not, as one might 

 suppose^ a product of the dissolution of the somites. 

 The latter do not contribute at all to its formation, it is 

 exclusively the little area of unsegmented mesoblast lying 

 originally in front of the first somite that grows out back- 

 wards and pushes below the scattered mesenchyme that has 

 resulted from the dissolution of the somites. In this way things 

 happen according to FRORIEP (1902, p. 44), but it seems 

 to me very difficult to conclude from sections, that this 

 view is the right one, and that it is not the dissolution of 

 the somites which is the source of the unsegmented mesen- 

 chyme mass. FRORIEP himself has been long in doubt, as 

 he admits, and I cannot feel convinced by his argumentation, 

 which is based on a slight difference in density of the 

 supposed two kinds of mesenchyme. 



Now the question is, whether the branchiomerism, which 

 soon after makes its appearance in this region, is quite 

 independent from the original mesomerism or if it corres- 

 ponds to it. In studying the work of the above cited authors, 

 one cannot avoid the impression that the adherents of the 

 first view place themselves on a too restricted stand-point 

 and that, to quote once more the words of ElSlG (cf. p. 58), 

 they are inclined to translate "eine ontogenetischeThatsache 

 willkurlich ins Phylogenetische." Truly, the correspondence 

 of branchiomerism to mesomerism is often, especially in 

 somewhat further advanced stages of development, far from 

 evident. As a rule, especially in higher Chordates, meso- 

 dermic segmentation is no longer to be observed in the 

 branchial region when the gill-slits have broken through. The 

 fact, however, that one gets the impression, that in ontogeny 

 the seriality of the gill-pouches originating from the entoderm 

 is the cause of the segmentation of the head-mesoderm, seems 

 to me not at all to exclude the possibility, that in phylogeny 

 the reverse may have been the case and that also in 

 ontogeny the place where the gill-pouches will arise has 

 been determined in an earlier stage by some influence 

 from the mesoderm, in Elasmobranchs e. g. by the somites 

 observed also by FRORIEP in the branchial region. One 

 of the strongest arguments for the latter view seems to 



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