306 EMBRYOLOGY 



though individuals show great variety, owing to various new chromosome 

 combinations. These new combinations of chromosomes, together with 

 mutations within the chromosomes, make for great variety in the individuals 

 developing from a single parent. These variations make possible an evolu- 

 tionary process, which over long periods of time results in new species. Thus 

 embryonic development does not reproduce the individual in exact detail. 

 The general characteristics are reproduced, but within this framework a wide 

 variety of individual characteristics appear. 



Summing up, we see that the complex adult develops from a simple egg 

 by a continuous series of changes so designed that each change stimulates 

 further differentiation. And this stepwise differentiation obviates the neces- 

 sity of any preformation hypothesis. Our analysis reveals development as 

 taking place by means of a series of changes resulting from stimuli. If we 

 attempt to outline this process, it can be done in a stepwise fashion beginning 

 as far back as is desired — for example, with light. Light stimulates the pitui- 

 tary. The pituitary then stimulates ovulation. The egg releases substances 

 which stimulate sperm activity. In turn, when the sperm reaches the surface 

 of the egg, stimulation of cell division takes place. During cell division and 

 early development the organizer is liberated and acts as the stimulus for the 

 formation of the neural plate. About this time other stimuli are released, so 

 that the sense organs — the ear, the nasal placode, and others — are stimulated 

 to form. In turn, the nervous system receives stimuli from such structures as 

 the limb. Development of the limb stimulates growth and differentiation of 

 the spinal nerves and the spinal cord. There are many other examples of 

 stimulation in later development. 



When all these phenomena have been reduced to chemical reactions be- 

 tween the stimulus and the tissue which is being stimulated, we shall have 

 a precise control of all developmental processes. In addition, if we permit 

 ourselves to accept the concept of a true dedifferentiation of cells in the 

 adult, then cells may return to an embryonic condition. If this is indeed so, 

 the repair and regeneration of parts of the adult organism may be controlled. 

 At present we have simply a general conception of this control. The outlines 

 are clear. And thus we may expect that future work will fill in the details. 



