PRIMITIVE STREAK TO SOMITE FORMATION 43 



The crescentic fold which bounds it is termed the head fold 

 and is the first definite boundary of the body of the embryo. 

 Throughout the course of development we shall find the head 

 region farther advanced in differentiation than other parts of 

 the body. This is a repetition of race history in the develop- 

 ment of the individual, for phylogenetically the head is the 

 oldest and most highly differentiated region of the body. It is 

 one of many manifestations of the law of recapitulation, in 

 conformity with which the individual in its development rap- 

 idly repeats the main steps in the development of the race to 

 which it belongs. 



