THE NORMAL SEQUENCE OF EVENTS IN EMBRYOLOGY 15 



the process of almost continual mitosis or cell division involving syn- 

 thesis and multiplication of nuclear materials. This continues through- 

 out the life of the organism but it is at its greatest rate during em- 

 bryonic development. 



Cell Differentiation (Specialization). 



After gastrulation the various cell areas, under new morphogenetic 

 influences, continue to produce more cells. However, some of these 

 cells begin to lose some of their potentialities and then to express cer- 

 tain specific characteristics so that they come to be recognized as cells 

 or tissues of certain types. Generally, after this process of differentia- 

 tion has occurred, there is never any reversion to the primitive or 

 embryonic condition. Differentiation is cytoplasmic and generally 

 irreversible. 



Organogeny. 



The formative tissues become organized into organ systems which 

 acquire specific functions. The embryo then begins to depend upon 

 these various newly formed organs for certain functions which are in- 

 creasingly important for the maintenance and integration of the 

 organism as a whole. 



Growth. 



This phenomenon involves the ability to take in water and food 

 and to increase the total mass through the synthesis of protoplasm. 

 Growth may appear to be quite uniform in the beginning. However, 

 as the organs begin to develop there is a mosaic of growth activity so 

 that various organ systems show peaks of growth activity at different 

 stages in embryonic development. It may be said that embryonic de- 

 velopment is completed when differentiation and organogeny have 

 been fully achieved. 



