5 • THE EARLY GASTRULA WITH 

 ITS PRESUMPTIVE VALUES 



A 



fter fertilization development takes place in a continuous progres- 

 sive fashion. There are usually no further stops in development. The concept 

 of stages in development is very misleading. Stages are convenient for descrip- 

 tion, but the transition from one stage to the next is so imperceptible that 

 what one actually sees is a process started in motion and continuously chang- 

 ing from minute to minute or from hour to hour, as the case may be. The 

 only sure method of identifying an embryo at any particular phase of its 

 development is by the time elapsed since fertilization at a specified tempera- 

 ture and in a specified environment. 



Study of the developmental process introduces a number of problems. 

 For convenience they can be grouped into two types. One is the problem of 

 differentiation, a process in the course of which cells in the embryo become 

 different types of cells in the adult. This is true development, a change from 

 the simple embryo to a more complex system. Simultaneously with this change, 

 with this differentiation, there is the process of growth, since the embryo 

 increases tremendously in size. This increase means that a synthesis of new 

 protoplasm is taking place either at the expense of the stored food reserves 

 or of food supplied by the mother. Growth, then, involves not only cell 

 division but also the reduplication of all the elements present in the original 

 cell, such as the nuclear materials or the various enzymes that function in 



55 



