SUMMARY OF DEVELOPMENT. 89 



3. It is a progress from relative simplicity to relative com- 

 plexity. The ovum is certainly vastly more complex than it 

 appears to the eye, but no one can doubt that the full-grown 

 worm is more complex still. 



4. It is a progress from a slightly differentiated to a highly 

 differentiated condition. The life of the ovum is that of a 

 single cell. The blastula is composed of a number of nearly 

 similar cells, which in the gastrula become differentiated into 

 two distinct tissues. In later stages the cells become differenti- 

 ated into many different tissues, which in turn build up different 

 organs performing unlike functions. 



5. Lastly, the development forms a cycle, beginning with 

 the germ-cell, and after many complicated changes resulting in 

 the production of new germ-cells, which repeat the process and 

 give rise to a new generation. All other cells in the body must 

 sooner or later die. The germ-cells alone persist as the starting- 

 point to which the cycle of life continually returns (cf. p. 73). 

 Their protoplasm, the " germ-plasm," is the bond of continuity 

 that links together the successive generations. 



