Page:Popular Science Monthly Volume 84.djvu/533

Rh maintains its own individuality. This fertilized egg fuses with no other cells, it takes into itself no living substance from without, 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 without 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 develops, step by step, without the addition of anything from without except raw materials and environmental stimuli, the multifarious

 Nereis, less highly magnified than Figs. 4 and 5, showing the progress of the turation spindle of the egg (1st Mat. Sp.); Fig. 7, the first polar body (1st PB) sperm nucleus and spindle (&#x2641;N); Fig. 9, the division of the male and female nuclei