THE GENERAL NATURE OF DEVELOPMENT 3 



with a multicellular organism. On the basis of some figures on the number of 

 cells in the brain, we can estimate that this single cell multiplies itself by 

 some thousands of billions of times. Both these changes, then, are of a tre- 

 mendous order of magnitude. 



The sperm cell, which is the stimulus for development, is smaller than the 

 egg. It is about 0.005 mm. in diameter, and weighs about 5 X 10 -9 g- Now 

 this cell must furnish the chemical compounds for all the hereditary char- 

 acters contributed by the male, such as the many physical features — color, 

 shape, size — and more complicated characters, such as abilities of various 

 sorts as well as many abnormalities, both in mind and physical structure. All 

 these hereditary factors must be contained in about five one-billionths of a 

 gram of protoplasm, much of which is water. Disregarding this water, we are 

 left with only a very small amount of solid material with which to work. 



Thus far we have considered the changes in size and number only. Were 

 it merely a matter of size and number, however, we could visualize how the 

 adult structure — given enough time and food and efficient catalysts in the 

 form of enzymes — could be built up by a chemical process. But there is in 

 addition a tremendous increase in structural complexity which occurs during 

 the development of the single cell into the fully formed organism. We 

 cannot measure this change in complexity, but we may illustrate it. 



Perhaps one of the most complicated single structures in the adult is the 

 eye. In the amphibian egg the eye can be traced back to a region which in the 

 early stages of development shows absolutely no structure whatsoever. The 

 protoplasm in that region is homogeneous under the most powerful micro- 

 scopes, and the cells which are derived from this protoplasm look alike, so 

 far as one can tell. Yet from this region, which is only a fraction of the 

 protoplasm of the egg, all the parts of the eye are derived. Thus, from what 

 visibly is no structure at all there develop the very complicated structures of 

 the retina, the lens, the cornea, and the optic nerve (Fig. 1). The magnitude 

 of the chemical and physical changes during the process of differentiation of 

 the eye are at least as striking as the changes in size and number during 

 development. 



Now, the study of the change of such small units as egg cells from a 

 simple to a complex condition is, naturally, difficult, and the historical survey 

 of embryology shows this clearly. It was only a little over a hundred years ago 

 that the human egg was discovered and that the proof that the human indi- 

 vidual came from a single egg in the follicle of the ovary was accepted. With 



