1*7.8 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



diverticulum becomes separated from the pharynx and commonly 

 gives rise to numerous rounded vesicles somewhat resembling those 

 of the thyroid in appearance. The organ thus formed was named by 

 van Bemmelen who discovered it in Elasmobranchs — suprapericardial 

 body — while Maurer has termed it the postbranchial body. Nothing 

 is definitely known regarding either its function or its evolutionary 

 history, though it is sometimes regarded as representing a vestigial 

 last gill-pouch. A curious point is the tendency of the organ to 

 unilateral development as it makes its appearance only upon the 

 left side in a large number of cases (Acanthias, Lepiclosiren and 

 Protopterus — see Fig. 109, B — most Urodeles, some Lizards). 



Fig. 100. — Ventral views of Polypterus larva to show the cement-organs. 

 A, Stage 30 ; B, Stage 33 ; c.o, cement-organ ; e.n, olfactory organ ; m, mouth ; V, ventricle of heart. 



known that the larvae of Actinopterygian ganoids possess cement- 

 organs on the head in front of the mouth. Balfour (1881) wrote of 

 this as " a very primitive Vertebrate organ, which has disappeared 

 in the , adult state of almost all the Vertebrata ; but it is probable 

 that further investigations will show that the Teleostei, and especially 

 the Siluroids, are not without traces of a similar structure." 



The organs in question were generally regarded as being developed 

 from a thickening of the ectoderm. Miss Phelps (1899) first stated 

 that they originated from endoderm (Amia) and the present writer, 

 at the time ignorant of her work, was greatly surprised to find 

 himself forced to this same conclusion by the examination of 

 Budgett's material of Polypterus. 



The cement-organ of Polypterus (Graham Kerr, 1906 and 1907), 

 when at the height of its development, forms a stout cylindrical 

 structure with a deep hollow at its free end, projecting from the 



