DEVELOPMENT OF HUMAN PHARYNX 377 



of the epiphysis, grows for a time, approximately during adoles- 

 cence, and then undergoes degenerative changes. The epiphysis, 

 however, is an encephalic evagination, primitively developing 

 an optic evagination, forming a parietal eye in certain lower 

 forms. The thymus I have just presented reasons for consider- 

 ing as formed out of the branchial epithelium by a kind of re- 

 action of degeneration attended by growth. The parathyreoids 

 are likewise formed out of the embryonic branchial epithelium 

 in its growth transformation. They are not encountered in forms 

 breathing through gills. The thyreoid of the lamprey^ — which is 

 the only form in which we obtain a glimpse of its origin — arises 

 out of the endostyle organ, when it degenerates at transformation 

 (Marine '13). It is customary, it is true, for zoologists to desig- 

 nate the endostyle organ as thyreoid. It may be pointed out, 

 however, that the endostyle organ corresponds in no respect 

 to the thyreoid that arises from it, in its morphology, and there 

 is, as far as I am aware, no evidence that the endostyle organ — 

 or, as a matter of fact, the thyreoid derived from it — possesses 

 the influence on growth characteristic of the thyreoid of mam- 

 mals. There is thus no more — nor as much — justification of 

 speaking of the endostyle organ as thyreoid than there would 

 be of speaking of the Graafian follicle as corpus luteum. It 

 develops very early in the mammal and is clearly an old organ. 

 The colloid vesicles, however, so characteristic of it, are more 

 comprehensible, according to Dohrn's view of the colloid as 

 stagnated and condensed secretion, primarily mucous, rather 

 than as exhibiting any adaptative feature correlated with its 

 influence on the bodily growth metabolism. 



The hypophysis is of more obscure genetic significance. We 

 have the attempts to interpret it as some form of ancestral 

 mouth (Owen, Beard, Dohrn, Kupffer) ; as primitively a sense 

 organ, possibly for the detection of changes in the sea-water 

 (Sajous); as preoral branchial clefts (Dohrn); as a gland (neural 

 gland) ; while the possibility is large that it may owe its develop- 

 ment at the end of the primitive growth axis and its curious 

 correlation with growth, to the more general growth conditions 

 of the region, rather than to any more specific phylogenetic 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 18, NO. 3 



