74 FRANKLIN P. REAGAN 



lost connection just as in normal heart formation. The part 

 of the concresced entoderm not utiUzed in the formation of the 

 pharynx has contributed to the formation of the abnormal 'body 

 wall.' So far as the body wall is concerned, the conditions in 

 this experiment are as if one should remove the yolk of a telo- 

 lecithal ovum and unite the cut edges of the ectoderm by a splice 

 of splanchnopleure. 



Figure 24 is a section through a trunk-meroplast. The inci- 

 sion on the right was rather close to the embryonic axis. On 

 the opposite side the tissue was left in communication with the 

 blastodemi. Concrescence from the left side has not only 

 reached, but has crossed the median line. The large heart is 

 really unilateral, exhibiting conditions closely resembling Graper's 

 figure 21. This tendency of the unilateral heart to assume 

 the dimensions of a normal heart is often very striking in em- 

 bryos in which one side is left in communication with the 

 blastoderm. 



e. A possible interpretation of certain 'mesothelial funnels^ and 

 multiple hearts. One of the most striking phenomena to be 

 observed in meroplasts containing the heart-region is the -reac- 

 tion of visceral mesoderm to the development of endothelial 

 tubes. One is impressed by the fact that where the vitelline 

 veins enter the body axis the visceral mesoderm is profoundly 

 affected. A most interesting problem is the determination of 

 the causal relationships involved. At present it is possible to 

 offer no more than a suggestion for the solution of this problem. 

 This much is certain: in the heart region of these meroplasts 

 excised on each side from the blastoderm, groups of cells become 

 proliferated between entoderm and splanchnic mesoderm; these 

 cells have a tendency to align themselves into solid cords or 

 hollow tubes, sometimes reticular or branched, sometimes singly 

 and sometimes separately as independent structures; correlated 

 with the presence of these structures there is a marked tendency 

 of the adjacent splanchnic mesoderm to attempt to enfold them. 

 Thus if a cord or tube lie longitudinally along the pharyngeal 

 wall, there is a tendency for the adjacent visceral mesoderm to 

 extend a longitudinal fold dorsal to the cord or tube, and a 



