178 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



di vertical u m bee.omes 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 lias 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 (Acantkias, Lepidosiren and 

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



CEMENT ORGANS OF TELEOSTOMATOUS FISHES. It has long been 



c.o 



B 



FIG. 100. Ventral views of Polypterus larva to show the cemeiit-orgaiis. 

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



known that the. larvae of Aetinopterygian ganoids possess cement- 

 organs on the head in front of the mouth. Balfour (1881) \\n>te "I 

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

 in the adult stair 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 Ljvnrralh regarded as being developed 

 from a thickening ,f the ret nderm. Miss 1'helps (IHD'.I lii>t stated 

 that they "i i^mated from endoderm (Aini<t) and the present, writer, 

 at thf time i^ii'.iant of her \\<>rk, was greatly surprised to tind 

 himself' forced to this same eonelusinn by the examination of 

 I'.ud. iierial of 1'nlypteru*. 



The c.-iiient OTgaO "I /'<>! ///>/' / x s ( ( \ ra 1 ia in Keri\ I'.MMjand 15)07), 

 uh'-ii at the height of its develcpnient.. InNns a stnin cylindrical 

 -tiueture with a deep hollow at its free end. prcjeetin^ fmm the 





