DEVELOPMENT OF HUMAN PHARYNX 333 



of the transformations of this region, whose morphology reflects 

 its primitive character as a respiratory apparatus. For the 

 skeletal elements of the region, the transformation history is 

 nearly though not completely clear; for the nerves, largely so; 

 for the musculature, less satisfactorily worked out. As to the 

 epithelial elements and the vascular elements, which are of 

 course the dominant tissues in the branchial respiratory apparatus 

 in lower forms, our knowledge is exceedingly crude; it covers 

 only the simplest morphology, while more recondite character- 

 istics that the , constituent elements may possess are unknown. 



From the references to the views of Peter and Kranichfeld 

 in the foregoing brief comment on the branchial pharynx and 

 the 'biogenetic law,' two views of the relation of embryonic 

 pharynx and 'pharyngeal derivative' are apparent — association 

 in development and fundamental genetic unity. It has been 

 intimated in the preceding paragraphs of this article that a 

 different interpretation may be advanced. This will be con- 

 sidered subsequently. At this point, I will venture to state 

 only that the biological significance and physiological effect of 

 the pharyngeal derivatives — thyreoid, thymus, and parathyreoids 

 — are w^apt up in the past history of the region and ultimately 

 explicable only in the light of their origin. This postulate I 

 conceive as applicable not merely to the structures in question, 

 but to all organs and all development. 



The general aspects of the problem of the embryonic pharynx 

 as outlined above determined my attitude in the study of its 

 developmental changes. I have furthermore, in attempting 

 to follow the growth transformations and shiftings, endeavored 

 to keep in mind, as far as possible, the original character of the 

 region and the fact that as far as the branchial epithelium and 

 vascular elements are concerned, it is a region whose primary 

 adaptative value is lost. 



While attention has been largely limited to the epithelium, 

 it became apparent that the epithelium could not be inter- 

 preted entirely by itself; that its growth transformations were 

 but a part of the growth changes in the region as a whole. It has 

 ever been the aim to keep the ultimate explanation of the mor- 



