in BKANCHIAL BUDS 177 



simply the modern representative of the endostyle of the proto- 

 chordate ancestor. 



An interesting feature is that while the physiological importance 

 of the thyroid in the modern Vertebrate is that of a ductless gland 

 for the production of internal secretion to be absorbed by the blood, 

 it still goes on producing the mucous material used by the far back 

 protochordate ancestor for entangling food particles, though that 

 substance is no longer, owing to the disappearance of the duct, 

 discharged into the pharyngeal cavity. 



Branchial Buds. — There make their appearance in the develop- 

 ing Vertebrate a series of bud-like proliferations of the endodermal 

 epithelium of the branchial clefts which may be known as branchial 

 buds. They appear at the upper and lower angles of the clefts and 

 the series shows its fullest development in the Lampreys, where buds 

 develop at the dorsal and ventral angles of all the clefts. In the 

 majority of fishes investigated they have been found to appear at 

 the dorsal angles of all the clefts except the first; in Urodele 

 Amphibians at the dorsal angle of all clefts and at the ventral 

 angle of II., III. and IV. ; in Anura at the dorsal ends of I. and II. 

 and at the ventral ends of II. -V. ; in Lacerta at the dorsal ends of 

 I.-III. and the ventral ends of III. and IV.; in Gallus at dorsal 

 and ventral ends of III. and IV. 



The morphological significance of these organs is still completely 

 obscure. Physiologically some of them appear to be of importance 

 during the later stages of development preceding sexual maturity 

 inasmuch as they give rise to that often bulky organ the Thymus. 

 This arises by the fusion together of more or fewer of the dorsal 

 buds, the others undergoing no further development. Thus in 

 Lepidosiren (Bryce, 1906) dorsal buds III. and IV develop into 

 thymus while II. and V. undergo no further development : in 

 Ceratodus (Greil, 1913) II., III. and IV. give rise to Thymus while 

 V. and VI. do not develop further : in ITypogeophis II., III., IV. 

 and V. give rise to Thymus while rudiments on I. and VI. atrophy. 



In regard to the much discussed histogenesis of the thymus all 

 that need be said here is that the originally solid epithelial rudiment 

 becomes in the course of development loosened out into a sparse 

 reticulum interpenetrated by mesenchyme richly traversed by blood- 

 vessels and crowded with leucocytes. 



The ventral buds, where they occur, become constricted off from 

 the branchial epithelium forming simple rounded masses of epithelial 

 cells (Amphibians) or they may be subdivided up by intrusive con- 

 nective tissue into solid portions (Beptiles) or hollow vesicles (Birds). 

 The small organs so formed are termed by their discoverer Maurer 

 epithelial bodies : their physiological significance is quite unknown. 



There normally develops in the Vertebrate either on both sides 

 or only on the left side a small pouch-like diverticulum of the 

 pharyngeal wall close to the ventral edge of the last gill cleft, what- 

 ever the number of this be in the morphological series. The 



VOL. II N 



