Developmental history of primary segments of the vertebrate head. 395 



The post-otic segments of Van Wijhe have been verified by 

 many observers who agree further that these segments are serially 

 homologous with the myotomes of the trunk. The pre-otic region, 

 however, constitutes a disputed territory. Rabl, '92, finds but three 

 segments in this region, while Dohrn, '90, has observed from twelve 

 to fifteen. The following brief review will show that there is also 

 lack of harmony regarding the morphological value of these segments. 



Ahlborn, '84, verifies the work of Van Wijhe with one ex- 

 ception — he makes one mesomere out of Van Wijhe's segments 3 

 and 4. He reaches the important conclusion that branchiomeres are 

 independent of mesomeres, and that the cephalic mesoderm presents 

 a typical metamerism "which developmentally is in perfect harmony 

 with the primary trunk somites". 



Gegenbauk, '87, considers Van Wijhe's head somites very dis- 

 similar structures. The 7th, 8th and 9th, he classifies as csenogenetic, 

 those cephalad to these he designates as palingenetic. He believes 

 there was originally complete concordance between branchiomerism 

 and mesomerism, but that subsequently, by a disappearance of some 

 branchial arches, trunk segments are projected forward. He discredits 

 the segmental value of Van Wijhe's first mesomere and is of the 

 opinion that the present number of cephalic segments gives no trust- 

 worthy indication of the ancestral history. 



According to Kastschenko, '88, the anterior trunk somite in 

 Selachian embryos is the first ontogenetic mesomere. To this, new 

 segments are added both caudad and cephalad, the latter growing 

 more and more indistinct towards the cephalic end where they finally 

 disappear in the vicinity of the first gill clefts. Between the first 

 and third gill clefts the evidence of metamerism is so indistinct that 

 the author is unable to determine the number of segments in this 

 region. He confirms the observation of Ahlborn regarding the in- 

 dependence of branchiomeres. He is also inclined to discredit the 

 morphological value of these "head cavities" as segmental organs and 

 says (in: Anat. Anz., V. 3, p. 761): "Although I do not agree with 

 Van Wijhe as to the morphological interpretation of the head cavities, 

 yet their ultimate fate, as described by this author, I completely con- 

 firm" (Author's translation). 



The following year Rabl, '89, expresses the view that the 5th 

 to 9th somites of Van Wijhe represent true metameres but believes 

 the segments cephalad to these are structures sui generis to which 

 must be assigned a different morphological value. In '92 the same 

 opinion is again expressed. 



