202 EMBRYOLOGY 



the principles formulated for the early embryonic development of the egg 

 apply to the later development of systems in the embryo. Labile organization 

 of parts followed by embryonic induction appear to provide a reasonable 

 interpretation of the development of individual structures. 



At this time it is well to pause to consider what has been accomplished 

 thus far and what has been omitted. We have gained an insight into the 

 embryological processes by which the several systems of the body develop. 

 We have as yet to consider the development of the reproductive system, and 

 we shall do so in the last chapter because we want to relate its differentiation 

 to the determination of sex, the maturation of the germ cells, and the begin- 

 ning of a new individual from a fertilized egg. And thus we shall have 

 completed the cycle from fertilized egg through adult to fertilized egg. 



Omitting, therefore, for the time being the discussion of the differentia- 

 tion of the gonads and having completed the treatment of the individual 

 systems, we now go on to a study of the embryo as a whole, using the chick 

 as material. 



