294 EMBRYOLOGY 



of human development, embryonic induction, although not proved by ex- 

 periment, is clearly indicated. 



Next, let us take up the question of fields. Do the various organs in the 

 body originate as fields which become smaller and smaller until they are 

 restricted to the development of a single organ ? One of the most character- 

 istic properties of a field is its ability to form two structures. Many descrip- 

 tions of development in the human embryo show that practically every organ 

 in the body is subject to duplication. Siamese twins and the doubling of arms 

 and legs or parts of the limbs or fingers are some examples. Indeed many of 

 the abnormalities which can be produced by operations on the amphibian 

 egg are actually observed in human development. For example, situs inversus, 

 a condition in which the organs are arranged in a pattern which is a mirror 

 image of the normal, can be produced at will in the frog embryo by reversing 

 the roof of the archenteron. Cyclopia, or a single median eye, is brought about 

 by chemical treatments of the fish embryo and is present in rare instances in 

 human embryos. Many other human abnormalities can be simulated by 

 experimental treatment of amphibian embryos. 



This brief survey of mammalian development brings us near the end of 

 our story of growth and differentiation. By careful observations, supple- 

 mented generously by experiments, we have arrived at a set of principles 

 consistent with the facts. We have derived the fully formed individual from 

 the egg. Now, to complete the story let us examine how the germ cells for 

 the next generation differentiate in the fully formed individual. We shall 

 thus bring the cycle of reproduction back to our starting point — the freshly 

 ovulated egg. 



