126 THE SCIENCE OF BIOLOGY 



hibits no foreshadowing of the adult body (Fig. 12 A). 

 When, however, the spermatozoon has entered (in this 

 instance the entrance occurs at some point on the equator) 

 differentiation begins. Stimulated by the entrance of the 

 sperm, the protoplasm changes its appearance to such a 

 degree that the axes and certain regions of the adult body 

 can now be recognized (Fig. 12 B). Portions of the egg 

 which later form the anterior, posterior, dorsal, and ventral 

 areas, are discernible and may be referred to the earlier 

 stage (Fig. 12 A). Areas from which are to be formed the 

 nervous system (n), primitive backbone (c), and germ-layers of 

 the early embryo appear within the egg, even before the 

 male and female nuclei have completed their union. Thus, 

 before any division of the egg-cell has occurred the general 

 regions of the adult body have been delineated (Fig. 12 A-I). 

 The cell divisions which ensue parcel out these predestined 

 regions to groups of cells which develop into the adult parts. 

 In some cases a few cells or portions of cells represent for the 

 time being an entire organ or group of organs (Fig. 12 E 

 and F). But this is not remarkable since the original 

 fertilized egg-cell or oosperm represents the entire adult 

 body. What is of interest for the present illustration is 

 that the steps from single-celled oosperm to many-celled 

 adult have been followed with such completeness. The 

 cellular continuity between generations is known to these 

 finest details. Embryological problems have, therefore, 

 been reduced to cell problems. 



But analysis of the cellular basis of development, and thus 

 of heredity, has been extended to structures and activities 

 within the individual cell. The basal fact of development 

 is that the many-celled adult arises from a single cell formed 

 by the union of ovum and spermatozoon. These germ-cells 

 are the most wonderful of cells, because of their potential- 

 ities. They are, as it were, vehicles of inheritance, by which 

 the characteristics of one generation pass to another. 

 Naturally enough, their structure and mode of origin has 



