THE ORIGIN OF THE HEART ENDOTHELIUM IN 



AMPHIBIA. 1 



J. B. JOHNSTON. 



The origin of the heart endothelium in Amphibia has been the 

 subject of several special investigations and of a considerable 

 volume of discussion. The question of fact may now be regarded 

 as settled. The work of Brachet has given definite and conclu- 

 sive evidence that the endothelium is derived directly from the 

 entoblast, as had been shown to be very probable by the earlier 

 work of Rabl and Schwink. The question now of interest is, 

 how is the derivation of the heart endothelium from the entoblast 

 in amphibia to be harmonized with its known origin from the 

 mesoblast in all other vertebrates ? The problem is that of the 

 homology of the heart endothelium of amphibia. Granted that, 

 as Ziegler contends, the condition in amphibia is to be regarded 

 as the result of ccenogenetic modification, exactly what is the 

 modification that has taken place ? What is the definite explan- 

 ation of the striking difference between amphibia and other ver- 

 tebrates ? As Brachet has pointed out, the term " ccenogenesis " 

 can not be invoked as a magic symbol to dispense with the whole 

 matter. It is not enough to say that in amphibia the endothelial 

 cells remain connected with the entoblast until a late period and 

 become separated after the mesoblast sheet has split off. This 

 offers no escape from the difficulty pointed out by Morgan ('97, 

 p. 151) that the heart endothelium must be considered to have a 

 different origin from the rest of the heart. 



The work upon which the present paper is based has been 

 done upon the eggs of a salamander which have been used for 

 class study for the past two years. The species has not been 

 identified because no adults have yet been taken. I hope at some ' 

 later time to give a description of these eggs and to deal with 

 some other features of the embryology of the species. The eggs 



1 Studies from the Zoological Laboratory of West Virginia University, No. 7 

 February 27, 1903. 



28 



