PHARYNGEAL DERIVATIVES OF AMBLYSTOMA. 



FRANCIS MARSH BALDWIN 



The recognition, within recent years, of the parts played by 

 certain structures arising from the walls of the human pharynx, 

 in health and disease, renders these bodies of great interest, and 

 hence all facts which throw light upon their morphology are of 

 value. It was from such considerations that the writer under- 

 took the study detailed in this paper, for, while a knowledge of 

 their development in amphibia may have no immediate practi- 

 cal value, the close relations of that group to the ancestry of the 

 mammals gives every addition to our knowledge an interest 

 which would not be expected with such groups as the teleosts 

 of the birds. 



The literature of the subject, which it is not necessary t.o go 

 into here, while not great, is rather difficult to follow, partly 

 because of the lack of unanimity of opinion as to homologies, 

 and partly because of the conflicting nomenclature of the sub- 

 ject, the same structures often having different names, even in 

 the minor groups of vertebrate animals. In some groups on 

 the other hand, jDliarv^ngeal derivatives are found which appar- 

 ently do not occur in the amphibians or at least in the urodeles. 

 These have not been considered in this study, and therefore this 

 paper is restricted to the consideration of the thyreoid and 

 thymus glands; the so-called postbranchial body; the so-called 

 'epithelial bodies' or 'gill-remnants,' and the carotid gland, 

 although the latter is not a true pharyngeal derivative. 



The study is based upon serial sections of larvae oiAmhly stoma 

 pimctatum, beginning with larvee 5nnn. long (about the time 

 of hatching), and including successive stages (8mm., 10mm., 

 12mm., 15mm., 19-20mm., 25mm., 35mm., 40mm.,) through the 

 45mm., following these stages with the transformation period, 

 and sections of young and old adult heads. Supplementing the 

 slides, gross dissections were made in the adult. The young 

 larvae were fixed in Smith's fluid, stained in toto in Borax- 

 carmine, dehydrated, embedded and mounted in the usual way. 

 The older lai-va^ and adult heads were fixed in formaline (heads 

 decalcified), dehydrated, embedded, sectioned and doubly 

 stained on the slide with Borax-carmine and Lyon's blue. Wax 

 reconstructions were made of the thyreoid gland in the 8, 9, 15, 



