HEART ENDOTHELIUM IN AMPHIBIA. 29 



have proved very favorable for study and the facts are so clearly 

 made out that they are thought to offer a solution of the problem. 



The earliest indication of the formation of the heart endothe- 

 lium is found in the rapid multiplication of the cells of the ento- 

 blast just behind the mouth anlage, at a period when the head is 

 slightly turned downward and before the gill slits have begun to 

 appear. As shown in Fig. i, the nuclei in this part of the ento- 

 blast are small, rounded, very numerous and closely crowded, 

 and many of them are in some stage of mitosis. The nuclei in 

 the remainder of the entoblast are larger and irregular, being 

 much distorted by pressure of the yolk grains, and mitotic fig- 

 ures are rare. The area described extends for a considerable 

 distance backward from the mouth, and the same conditions pre- 

 vail on the cephalic surface and the sides of the pharynx close to 

 the mouth anlage. Rapid growth in these latter regions con- 

 tinues later than behind the mouth and is connected with the 

 formation of head mesenchyme. The region of growth behind 

 the mouth is noticeable in both transverse and sagittal sections, 

 but it is of short duration and in slightly later stages the cells are 

 relatively larger and the nuclei have the appearance of resting 

 nuclei. At the point nearest the mouth the cell divisions con- 

 tinue until the time of separation of the heart endothelium. 



The formation of the mesoblast and its early differentiation 

 furnish the facts of greatest significance for our problem. In the 

 head and anterior part of the trunk the mesoblast is split off from 

 the entoblast to a point some distance from the mid-ventral line, 

 where the delamination appears to stop. That this is a definite 

 limit beyond which delamination does not go is evidenced by the 

 distinct separation between entoblast and mesoblast which often 

 occurs even in very early stages (Fig. 2, a and b\ by the total 

 absence of nuclei in the outer half of the entoblast ventral to the 

 limit mentioned, and by the future history of the ventral portion 

 of the mesoblast. Mitotic figures often appear very early in the 

 ventral edge of the mesoblast sheet (Fig. i), and although they 

 do not appear in the sections drawn in Fig. 2, they are usually 

 more numerous there than elsewhere in the mesoblast. The 

 result of rapid growth here is to cause a decided thickening of 

 the ventral edge of the mesoblast, and in this thickening the body 



