GROWTH 



size of a pinhead, grew into a giant Sequoia, which now stands 

 on the California coast, with a trunk thirty feet in diameter at 

 the base and towering three hundred feet into the air. During 

 these three thousand years the original single-celled zygote has 

 divided into an almost incalculable number of cells, for a cubic 

 inch of Sequoia wood contains over one billion. During the 

 same period differentiation resulted in the formation of the 



roots, the trunk, branches, 

 needles, cones, and other parts 

 which by their shapes and ar- 

 rangements enable us to recog- 

 nize this tree and enable the 

 tree to continue its life and 

 growth. Few of the innumer- 

 able cells which now make up 

 the various parts of the mature 

 tree resemble the ellipsoidal, 

 thin-walled, colorless cell 

 which was the zygote. At the 

 stem tips and root tips and in 

 the cambium layer there are 

 still embryonic cells which 

 continue dividing and which 

 resemble in some respects the 

 original zygote from which 

 they came. The others have differentiated into cells with a va- 

 riety of shapes, sizes, thicknesses of wall, types of content, and 

 they now constitute the characteristic and functional units of 

 the different parts of the tree. 



The vertebrate animal, in common with almost every other 

 living thing, also originates as a single cell, the zygote or fer- 

 tilized eggj which divides into many cells at first much alike in 

 general shape and contents. In Figure 4 the early growth of a 

 vertebrate animal, a salamander, is pictured. The first drawing 

 represents the single-celled zygote, microscopic in size. At that 



Figwe 4. The growth of a salamander, 

 Ambly stoma punctatum, from zygote (No. 

 1) to many-celled sphere (No. 9). After 

 Eycleshymer. From Minot's Age, Growth 

 and Death. By permission of G. P. Put- 

 nam's Sons. 



