ABSTRACT
The concept of a critical period was introduced into the embryological and biological literatures some 8 decades ago when Hans Spemann (1901), studying amphibia, demonstrated that grafts from one portion of the egg developed the characteristics of the host tissue. Tissue, that if left alone, would have developed into ventral surface, for example, developed into an eye cup when grafted to that designated area of the host. The reverse, of course, also held. When these surgical manipulations were undertaken sometime later in development, the donor tissue grew into the designated donor organ, regardless of where on the host’s body the donor tissue was grafted (Mangold & Spemann, 1927).
