The majority of studies devoted to animal development traditionally start out from questions of morphogenesis. Of course, visible differentiation, as well as the events leading to it, should ultimately become describable in molecular terms. Nevertheless, even simple morphogenetic processes may have a complex biochemical basis which makes it difficult to recognize the key functions involved. This difficulty obviously does not exist in the case of glands, i. e., organs and tissues primarily concerned with, and characterized by, the nature of their products, i. e., one, or a few secretory...