HEART ENDOTHELIUM IN AMPHIBIA. 3! 



cavity early makes its appearance (Fig. 3, a). With further 

 growth the body cavity enlarges and the entoblast is laterally 

 compressed between the cavities of the two sides. As a result, 

 the growing entoblast behind the mouth, above described, takes 

 on the form of a keel. Later the body cavity (pericardial cavity) 

 spreads ventrally and mesially, and the mesoblast insinuates 

 itself between the heart endothelial cells and the ectoblast and 

 later between these cells and the entoblast. This movement is 

 due entirely to the growth and spreading of the mesoblast earlier 

 split off and not to a further delamination from the entoblast. 

 There is no sign of any further delamination of mesoblast after 

 the stage shown in Fig. 2, but on the contrary the mesoblast 

 grows continually more and more sharply distinct from the 

 entoblast after that period. The pushing down of the mesoblast 

 in the region of the heart, which accompanies the enlargement 

 of the pericardial cavities, is well advanced while the thickened 

 ventral edge of the mesoblast farther caudally has not shifted its 

 position (Fig. 3, a, b, <r). The region in which the delamination 

 of mesoblast does not reach the mid-ventral line extends caudally 

 to a point a little behind the middle of the embryo and this region 

 probably includes the blood island described by Brachet. The 

 writer has not yet fully investigated this region, but if the surmise 

 here made is correct, the reasoning applied to the question of the 

 heart endothelium will apply equally well to the blood island. 

 To recapitulate, there is a mid-ventral area or keel of entoblast 

 extending backward from the mouth anlage, from which no 

 mesoblast is split off in the species studied. From this area the 

 heart endothelium (and perhaps the blood) are formed. 



A second fact of some interest for us is that the mesoblast 

 shows a tendency to split off late, so that it is already divided 

 into regions when it first separates from the entoblast. This is 

 seen especially in the formation of the mandibular arch. As 

 shown by Fig. 2. e, the mandibular arch mesoblast, at its first 

 appearance is separated from the rest of the mesoblast by the 

 first gill slit, and it never has any connection with the mesoblast 

 bounding the pericardial cavity. Indications of the second gill 

 slit also appear very early, so that in some cases the hyoid arch, 

 which is continuous with the pericardial cavity, seems to be split 



