24 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



uterine development. Ontogeny is a broader term, 

 which includes not only embryology, but the develop- 

 mental history of an individual .up to old age or the 

 senile condition. There is another form of develop- 

 ment, much slower, but just as certain as ontogeny. 

 This affects not only the individual, but, collectively, 

 every member of the animal group. In the study of 

 races there is ample evidence that structural changes 



Fig. i. Formation of the polar bodies in the ova of Asterias gla- 

 cialis (Hertwig): ps, polar spindle; pb' t first polar body; pb", second 

 polar body; n, nucleus returning to condition of rest. 



have slowly but gradually taken place. This 

 broader developmental history, or history of a race, 

 is known as phytogeny. It is closely interwoven with 

 the ontogenetic development, so much so, that the 

 latter in large part repeats the former, or one's 

 phylogenetic history is repeated in the ontogeny. 



In development, therefore, the phylogenetic or 

 intrinsic qualities of a cell are important factors. 

 These factors constitute heredity. There is further 



