616 



fifth pouch, since it is or appears to be already present in the two 

 embryos in which a fifth pouch is shown. Nor does it seem to me 

 that there is any better reason for describing it as an appendage of 

 a fifth pouch. Into it, in later development would undoubtedly go 

 the cells which actually took part in a fifth ento-ectodermal contact, 

 or might have done so had it been developed. 



As representing a continued growth in the entoderm of the 

 " branchiogenic " region it might possibly be described as embodying 

 a fifth and sixth poach, a seventh or possibly even more potential 

 pouches. But this expression of its significance would, I believe, fail 

 to pass scrutiny if interpreted as meaning that the potentialities of 

 these pouches were intrinsic in groups of its cells. The Branchio- 

 meric Organ concept of the pharyngeal derivatives must see in this 

 structure the representative of a definite pouch, or an "organ" 

 belonging to a definite pouch. It should not be forgotten, however, 

 that this is a point of view, an a priori assumption whose value in 

 explaining the existence of such structures as the organs arising out 

 of the pharyngeal transformations is yet to be determined. 



If all theoretical interpretations are left out of consideration, and 

 this structure is presented simply in terms of the growth transformations 

 of this region, it appears that it is an expression of a continued 

 growth tendency in the pharyngeal entoderm apparently associated with 

 the gill-forming potentialities of the region, in all probability correlated 

 with the more extensive branchial apparatus of lower and ancestral 

 forms. The interpretation of the Biogenetic Law here intrudes itself. 



Furthermore, I can see no adequate reason for considering the 

 boundaries and limits of branchial pouches as intrinsic in their cells 

 rather than established in the differential growth of the entire branchial 

 region. I believe that the pouch boundaries as such have no intrinsic 

 significance in the later transformations of the caudal pharyngeal 

 complex, that is of the fourth and (if it comes to development) fifth 

 pouch and of the so-called ultimobranchial body. This opinion will 

 be considered subsequently. It is obvious from the above that I re- 

 gard the name ultimobranchial equally with that of postbranchial as 

 unfortunate and some day to be replaced by a more appropriate term, 

 if ' ultimobranchial ' is to be employed with other than a mere des- 

 criptive force. 



In the 6.25 mm. embryo, and particularly in the one 7.5 mm. 

 in length, the pharynx of which was modelled, the ultimobranchial is 



