in CEMENT-OKGANS 181 



Amia is of special interest in regard to its cement-organs as it 

 was in this form that their endodermal origin was first announced. 1 

 The organs are for a time in the form of a pair of rounded knobs, 

 one on each side, but these take on a crescentic shape so that together 

 they form a circular wall, interrupted anteriorly and posteriorly. 

 Each organ contains a pocket-like projection of the gut-wall which 

 takes on a somewhat sausage-like form in correlation with the curved 

 shape of the organ as a whole. This endodermal sac separates from 

 the main endoderm and becomes constricted across, so as to form a 

 curved row of closed vesicles from six to ten in number. Each 

 vesicle fuses with the ectoderm and develops an opening to the 

 exterior so that it takes on the appearance of a cup at first deep and 

 narrow, later shallow and wider, its lining continuous with the deep 

 layer of the ectoderm. 



When the larva reaches a length of 13-14 mm. it makes less use 

 of its cement-organ and the latter commences to degenerate, sinking 

 beneath the surface with which, however, it remains connected by 

 a narrow tubular channel. By about the 20 mm. stage this has 

 disappeared and soon there is no trace of the organ to be found even 

 in sections. 



In Zepidosteus the organ appears to be similar while in the other 

 ganoids its development still remains to be worked out. 



These cement-organs are of special interest and importance for 

 more than one reason. In the first place they are of importance in 

 revealing a quite unexpected pitfall in the way of the investigator 

 trained to have implicit faith in the germ-layer theory, for they show 

 how a particular organ may become transferred from one germ-layer 

 to another even though not belonging to the transitional zone where 

 the two layers are continuous. A very common modification of 

 ontogenetic development consists in the slurring over or even 

 omission of particular stages in early development. Were this to 

 happen in the case of the early stages in the development of the 

 cement -organ say of Polypterus, it is easy to see that the organ 

 might have every appearance of being purely ectodermal in its 

 nature, although it is, as a matter of fact, endodermal. 



It appears to the present writer quite possible, if not probable, 

 that this modification has actually come about in the Dipnoi and 

 Amphibians, and that the cement-organs of these groups, although 

 they develop from the ectoderm in those forms which have been 

 investigated (p. 79), are really homologous with the cement-organs 

 of the Teleostomi, their endodermal stage having been eliminated 

 from ontogenetic development. Further investigations are needed 

 in the Amphibia — to see whether no trace exists, in any member of 

 the group, of an original connexion with the endoderm. 



As regards the original nature of these organs it is impossible 

 to arrive at any certain conclusion. Arising as they do in the form 



1 Phelps (1899). The actual discovery seems to have been made by^ Reighard. 

 Cf. Reighard and Phelps (1908). 



