ORIGIN AND STRUCTURE OF THE HEAD 69 



is provided by the craniovertebral limit, the correct deter- 

 mination of which in succeeding stages he guarantees, 

 though with the somewhat restricting addition "(Irrtum 

 vorbehalten)". 



Thus FRORIEP reaches the conclusion that a consider- 

 able part, nay, by far the greatest part, of the branchial 

 region originally indeed exhibits segmentation, though 

 secondarily this is destroyed and subsequently replaced by 

 the branchiomerism, of which only the latter would have been 

 observed by VAN Wyhe and ZlEGLER, who studied stages 

 much too far advanced in development. We get the impres- 

 sion, that it is no easy task for FRORiEP to demonstrate 

 that the auditory vesicle still belongs to the unsegmented 

 region in front of the first somite, since in his own drawings 

 we see the series of somites clearly reaching to in front of 

 the auditory vesicle. One would be inclined to ask if, 

 in such Vertebrates, where no prootic somites are to be 

 observed, the reduction demonstrated by FRORIEP in Elas- 

 mobranchs could not have already set in in the earliest stages 

 observed and even before the somites become evident ? 



Main points on which opinions differ. — At any rate we 

 can state that the main differences between the two concep- 

 tions prevailing until the present day is reduced to a diver- 

 gence of opinion on the following questions : 



1. Is there a part of the head that is primarily unseg- 

 mented and is there a primarily unsegmented head mesoderm? 



2. Does the auditory vesicle belong to this unsegmented 

 head-region or to the region of the somites? 



3. Does the branchiomerism correspond to the meso- 

 merism and do the gill-slits belong to the first or to the 

 second region mentioned under 2? 



Unsegmented region of the head. — As regards the first 

 question, both parties recognize the existence of an anterior 

 part in which no segmentation can be traced. It is the ever- 

 tebral region ofGEGENBAUR — corresponding, together with 

 the mandibular segment, to the acromerite of Hatschek 

 (1909, 1910) — resp. the praespinal or cerebral region of 

 FRORIEP, which reaches to the foramen of the vagus. The 

 praechordal part, by the adherents to the first view, how- 

 ever, is generally considered as a secondary outgrowth from 

 the first segment, their starting point being accordingly 

 a body uniformly segmented from the tip of the nose 

 to the end of the tail; the foremost segments having 



