615 



strictly speaking a sixth poucii. In the next embryos (6.25 mm., 

 7.5 mm.) the ultimobranchial body attains a large size, relatively 

 largest at about this period of development. 



The mode of development of this structure indicates — to me it 

 seems clearly — that it, as a growth, belongs to the branchial portion 

 of the pharynx. The branchial pouches and clefts are differentiated, 

 as is of course well known, in a cephalo-caudal succession following 

 the law of differential growth characteristic of all the segraentally 

 arranged structures of the vertebrate body. This applies to the visceral 

 and vascular arches as well. It would thus appear that each branchial 

 pouch is, when first formed, more closely associated with the pouch 

 immediately cephalad of it. It is the growth of the arch (and later, 

 that of its derivatives) which together with the growth of the ento- 

 dermic epithelium determines the form and relations of the pouch. 

 Where an arch remains undeveloped or vestigial the succeeding pouch 

 of necessity appears as an apparent outpocketing of the preceding 

 pouch. There have been found, as far as I am aware, but two human 

 embryos in which the pharyngeal entoderm in the process of its 

 growth reaches the ectoderm again caudad of the branchial membrane 

 of the fourth pouch, — the Buxton 199 embryo of 5 mm., as above 

 briefly described, and the 5 mm. embryo described by Hammar ('04; 

 Keibel & Eltze '08, N. T., JN"o. 20). In both the contact occurs on 

 one side only, holds in a single section (Keibel & Elze, '08) and is 

 equally small in the embryo examined by me, so slight in the Hammar 

 embryo that it was overlooked in what was apparently an earlier 

 description of the pharynx (Hammar, '01) and was not recognized 

 by me in the Buxton embryo until a model was constructed, the 

 plane of section being particularly unfavorable. There w^ould appear 

 to be little reason to reject this contact as constituting a vestigial fifth 

 pouch. Whether or not it is at all constant in its appearance is a 

 question about which there may well be a difference of opinion and 

 considerable justifiable doubt. 



Continued growth produces the blind pocket of pharyngeal ento- 

 derm termed the ultimobranchial body. It may be described as 

 produced by a continuation of the growth process in the pharyngeal 

 entoderm, which as part of the differential growth of the region has 

 formed the successive branchial pockets. In its development it would 

 from this point of view be linked thus with the branchial region. 

 It could hardly represent in any morphological sense a rudimentary 



