This investigation was carried on under the direction of Prof. B. F. Kingsbury to whom I wish to make grateful acknowledgment.
300MELVIN CEAWFORD GODWIN applied the principles of Mangold's ( '22) experiments on induction of indifferent embryonic material to the interpretation of the ultimobranchial transformation. He regarded the ultimobranchial body as a natural transplant of essentially indifferent material which under the influence of the thyroid was induced to also become thyroid. Although this interpretation is a very plausable one the exact details of the transformation present certain difficulties. An attempt has been made in this study to follow the histological process of follicle formation from ultimobranchial material in the postnatal animal.The problem of the thymus-particularly thymus I V -e a nnot be omitted from an interpretation of complex 'IV or of the ultimobranchial body. Stewart ( '18) and later Mason ('31) found the ultimobranchial body in the cat to become cystic or undergo a thymic transformation. DeWiniwarter ( '35) working on the cat finds the ultimobranchial body persists but does not form thyroid tissue. He regards the ultimobranchial body as a fifth pouch. Indeed in the guinea pig ('33) he finds that the ultimobranchial body forms a parathyroid V and thymus V quite constantly while the fourth pouch disintegrates as a rule but may at times form a parathyroid N and thymus IV. Such a condition if true would provide a material basis for the specific pouch homology of Getzowa ('11) who interpreted the ultimobranchial body as a fifth branchial pouch having parathyroid and thymus forming potencies. It is interesting to note that Rogers finds no fourth pouch at all in the rat while Selle ( '35) reports a complete absence of the ultimobranchial body in the pallid bat. Also of interest is the recent revival by Weller ('31) of the term lateral thyroid for the ultimobranchial body in the human and with it the speculation that it is endowed with thyroid forming potencies. Kingsbury ('35 a) has shown that the evidence does not justify such a conclusion. Neither of these authors find a thymus IV in their material but it seems certain that a thymus IV at times is present in the human. The evidence in the dog indicates that the ultimobranchial body is not endowed with thyroid forming potencies ULTIMOBRANCHIAL BODY AND THYROID 301 but that it does form thyroid tissue by induction. Thymus IV is irregularly present in the dog and is apparently derived from that portion of complex IV originally present in the fourth pouch. Kingsbury ('35b, '36) finds the thymus I V in the calf does not bear any significant or specific relation to pouch IV. In the same form Kingsbury ('35 b) finds a portion of the complex persists as highly chromatic epithelioid cords and does not form thyroid or undergo thymic transformation. The whole of pouch I11 of the calf becomes thymus (Anderson, '22) excepting of course parathyroid I11 but the whole complex I V does not form thymus in any instance. It is the caudalmost portion appa...
