The distribution of thyroid tissue in chorioallantoic grafts from presomite chick blastoderms has been analyzed in a previous communication ( '32). I n primitive streak stages, grafts of median (prospective dorsal) strips contain thyroid as frequently as do lateral (prospective ventral) pieces. The number of cases in which thyroid differentiates from median pieces decreases markedly in head-process stages. The same is true of liver differentiatiation (op. cit. table 1). Thus in these cases an increasing specificity of histogenetic potency accompanies the early differentiation of the embryonic axis. Respiratory tract epithelium, conversely, shows no tendency during these stages to become localized in prospective ventral regions ( '33).
The chick chorioallantois, used as a substratum for testing potencies of embryonic grafts, has certain peculiar properties. Nutritional and mechanical conditions are adequate to permit complete cytological and histological differentiation, and indeed to allow extensive overproliferation of some elements (Nicholas and Rudnick, '33). Larger-scale events, organogenesis and embryogenesis are to a great extent suppressed; probably by mechanical conditions. The rat embryo (op. cit.) grafted to the chorioallantois may either differentiate histologically as an incoherent mass of tissues, as do chick embryos, or may persist as a morphological whole, in which case differentiation is slight. Controlled, organized development is, then, not possible on the chorioallantois.
' National Research Council Fellow.