HEART ENDOTHELIUM IN AMPHIBIA. 33 



which have recognized the derivation of the endothelium directly 

 from the entoblast, the mesoblast sheets were split off first, and 

 later consequently from deeper layers of entoblast the cells 

 destined to form the heart endothelium were split off. Since in 

 other vertebrates only one layer of cells is split off and the heart 

 endothelium is differentiated from a part of this mesoblast, the 

 conclusion that the endothelium of amphibia has a different 

 origin from that of other vertebrates was unanswerable. In the 

 species studied by the writer (and also, apparently, in those 

 described by Brachet) the mesoblast sheets are split off earlier 

 from the entoblast except in the region in which the heart 

 endothelium will appear, and later the endothelium is split off 

 from a part of the entoblast which has not given rise to any 

 (other) mesoblast. Therefore, in these forms, the heart endo- 

 thelium is derived from the same source as the mesoblast sheets, 

 namely from the superficial layer of entoblast, and the difference 

 between these amphibia and other classes of vertebrates consists 

 only in a somewhat general tendency for the mesoblast to split 

 off relatively late and to be marked out into definite organs, or 

 organ-anlages, at the moment of splitting off. This is seen not 

 only in the splitting off of the heart endothelium at a little later 

 time and separately from the rest of the mesoblast, but also in 

 the same mode of formation of the mandibular and hyoid arches 

 and of a part of the head mesenchyme. The writer believes that 

 a reexamination of other amphibia, at least of Urodeles, at the 

 proper stages of development will show the process here described 

 to be characteristic for amphibia. In brief, then, the heart 

 endothelium of amphibia is strictly mesoblastic, although it is not 

 at any stage identified with the undifferentiated mesoblast, being 

 split off from the entoblast in the same manner as the rest of the 

 mesoblast, but somewhat later and separately. 



DESCRIPTION OF FIGURES. ABBREVIATIONS. 



a., aortic arch cells; arch., archenteron ; br.i, first branchial arch; c., ccelome ; 

 fff., ectoblast; en/., entoblast; g.i, g.2, g. J, first, second, and third gill slits; /;., 

 heart region ; h.e., heart endothelium ; ky., hyoid arch ; /., liver region ; m., site of 

 mouth ; m.a. , mandibular arch ; rues. , mesoblast \p.c. , pericardial cavity ;ph., pharynx. 

 Small crosses indicate the position of mitotic figures. In Figs. I and 4 resting 

 nuclei are shown as black spots. 



