Although our knowledge of the primordia of the mammalian heart is based largely upon descriptions of fixed material, recent observations of living embryos cultivated 'in vitro' by Nicholas and Rudnick ('34)' Waterman ('33) and Waddington and Waterman ( '33) have made valuable contributions. Since these authors did not study the potentialities of the heart rudiment specifically and since there are certain differences between the development of this organ in mammals and lower forms, their method has been adapted in the following experiments to a closer study of this problem.
The origin of the heart from two laterally situated primordia has been established by comparative studies of lower forms and by experimental work performed upon them. Graper ('07) intrigued by Verocay's report of a chick with seven hearts, devised an ingenious experiment to test the potency of the lateral anlagen for individual development. A tiny blade was held in place over chicks of 28 to 30 hours' incubation in such a way that it pressed down upon or just behind the head. After a further incubation of 18 hours, Graper reports the development of two hearts, one on each side of the embryo, both of which he considered to be morphologically adequate and probably functioning organs. I n Amphibia, the developmental potency of the heart has been subjected to a more extensive experimental analysis. Ekman ( '24) and ( '25) split the heart rudiment in anuran embryos and obtained 33