Such processes, and those of the other elements of the body, which have to do with the changing constituents of the human organism, are all cellular processes--metabolism. Hence the chief organic act of the body is metabolic; the basic organ of man is the cell. Arms, legs, heart, brain, stomach, and similar organs are secondary, though some of the latter are essential for certain operations. Now, one cell is an organ amply sufficient for metabolism, for the chief organic act of the body; hence it is a fitting receptacle for a substantial form, a soul. Therefore there is no reason why the soul may not be present in the one-cell stage of the embryo; and since there is no reason why it should not be present, but many why it should, it is present. Conklin says:[53] "The fertilized egg of a star-fish, or frog, or man is not a different individual from the adult form into which it develops, rather it is a star-fish, a frog, or a human being in the one-celled stage. This fertilized egg fuses with no other cells, it takes into itself no living substance, but manufactures its own protoplasm from food substances; it receives food and oxygen from without and it gives out carbonic acid and other waste products; it is sensitive to certain alterations in the environment, such as thermal, chemical, and electrical changes--it is, in short, a distinct living thing, an individuality. Under proper environmental conditions this fertilized egg-cell develops, step by step, without the addition of anything from the outside except food, water, oxygen, and such other raw materials as are necessary to the life of any adult animal, into the immensely complex body of a star-fish, a frog, or a man. At the same time, from the relatively simple reactions and activities of the fertilized egg there develop, step by step, without the addition of anything from without except raw materials and environmental stimuli, the multifarious activities, reactions, instincts, habits, and intelligence of the mature animal." [53] _Heredity and Environment._