611 



pouch. While this interpretation satisfies the requirements of a 

 homology as between the higher vertebrates and the amphibia, it 

 fails as applied to the elasmobranchs where the sixth pouch is a 

 functional gill cleft caudad of which occurs the suprapericardial body 

 which appears to be an ultimobranchial body (van Bemmelen '89, 

 Greil '05). 



For those, who vie^sv this structure as a vestigial ancestral gland 

 of some kind, Maurer's term and the interpretation inherent therein, 

 — Postbranchial body, — present no such logical difficulty, since, as 

 Maurer ('11) said in defense of his term at the Leipzig meeting of 

 the Anatomische Gesellschaft, these structures might then be homo- 

 logized throughout the vertebrate series in the forms in which they 

 occour. Such was the conclusion of Verdun '98, as a result of his 

 extensive studies, unfortunately, however, the evidence, — at least as 

 the present writer sees it, — indicates strongly that the structures in 

 question, in the vertebrate series, belong to the branchial region or 

 series and are not "postbranchial". 



The only remaining alternative whereby these pharyngeal structures 

 may be interpreted as ultimobranchial and also directly homologized 

 in the different vertebrates, would seem to be the assumption, — purely 

 gratuitous it is true, — that it is the last branchial pouch which in the 

 form of the ultimobranchial body or represented by it as a derivative 

 has retained its individual existence while the reduction in number 

 of branchial pouches has been brought about by the elimination of 

 gill clefts that preceded it in the series i). It would then be rather 

 difficult to determine its proper serial number which would at least 

 be number VIII. Such an interpretation has little claim on our at- 

 tention. Disregarding for the time a comparison with the lower ver- 

 tebrates and confining the consideration to the condition in mammals, 

 there would be the choice in interpretation between a derivative of 

 a fifth pouch and a representative of a sixth pouch. Consider- 

 able evidence indicates the existence in Amphibia of six potential 

 pouches cephalad of the lungs. Whatever view be taken of the origin 



1) DoHEN presented the hypothesis that the thyroid represented a pouch 

 which had originally occurred between the hyomaudibular (I) and first gill 

 cleft (II). MiNOT (Laboratory Textbook of Embryology) states that some 

 authouties maintain that a pouch has been lost, between the present third 

 and fourth pouches. I know of no clear evidence supporting either of these 

 contentions. 



39* 



