An Analysis of the Postgastrula Differentiation of the Hypomere

Following its involution around the lateral lips of the blastopore, the potential hypomere (lateral plate mesoderm) of Urodeles undergoes a dorsal convergence, along with other portions of the mesoderm mantle and then, during neurulation, initiates an independent ventrad migration (Nieuwkoop, 1947). As the hypomere migrates ventrally, it becomes further removed from the influences shown by Yamada (1950) to emanate from the mid-dorsal axial system and its deep layer is in contact with a tissue (endoderm) which seems to be influential in the differentiation of this mesoderm (Bacon, 1945; Jacobson, 1960; Nieuwkoop, 1947, 1950; Finnegan, 1953, 1955, 1961 a , 1961 b ). A further understanding of the postgastrula development of the trunk hypomere seemed to require information of the possible restriction, in time, of the competence of this mesoderm or, alternatively, its degree of self-differentiation within an embryonic system for in vitro observations may indicate histogenetic potencies unrelated to normal development (Finnegan, 1961 a , 1961 b ).