ONTOGENY 19 



which are to be built into the structures of the simple embryo, 

 and various differentiated substances of the egg are segregated 

 among different groups of cells. Following this are the phases 

 of blastula formation, when the cells become arranged in a 

 definite layer, and then gastrula formation when the cells are 

 rearranged into two definite layers. Then comes the period of 

 embryo formation, when the cells of the layers are moulded into 

 the earliest beginnings of the chief systems and organs, blocking 

 these out in the simplest manner. During this last phase 

 growth becomes very rapid, accompanied by continued cell 

 division, no longer termed cleavage, and as the formation of 

 organs becomes more complete and more particular, the embryo 

 increases in bulk and dimensions. This period of embryonic 

 development may occupy a long time, and usually leads to 

 the formation of an organism which is capable of leading an 

 -independent life, either as a larva or as a form closely resem- 

 bling the adult, except in size. Finally, accompanied by con- 

 tinued growth, the last phases of development appear as cellular 

 differentiation becomes more complete, and the organism begins 

 to assume more fully the characteristics of its parents. When 

 the reproductive tissues become functional as such, the animal 

 is considered mature and its development complete, although 

 in a true sense development is never entirely completed, for 

 the form of the organism never becomes definitely fixed, and 

 cellular differentiation seems never to cease during the life of 

 the organism. 



It is important to remember that all of these phases of 

 development are continuous and more or less overlapping, and 

 in all of them, excepting perhaps the earlier, where the impor- 

 tant changes concern chiefly the structure and the composition 

 of the germ nuclei, the three processes growth, cell division, 

 and differentiation are going on together. Yet in general it is 

 clear that in the early stages of development, after the gametic 

 nuclei are differentiated and fused, cell division is the process 

 of greatest activity; then follow stages during which develop- 

 ment is characterized chiefly by growth; and lastly the final 

 aspects are chiefly the result of cellular or tissue differentia- 



