182 LIFE: ITS NATURE AND ORIGIN 



nevertheless is of great importance when the fertilized ovum or 

 zygote develops into an individual — a process dubbed ontogeny. 



The actual fertilized ovum in the hen's egg, with which we are 

 most familiar, is also very small, the main mass of the egg (its 

 yolk and white) consisting of water and nutrient material suffi- 

 cient to enable the germ to develop into an individual when the 

 egg is cast off or "laid" by the hen. Such a self-contained egg is 

 termed cleidoic (closed box), for it has within its shell everything 

 it needs, except some atmospheric oxygen which can diffuse in, 

 to supplement the air bubble supplied to the egg by the hen. 

 But in some of the lower animals (e.g., sea-urchin, squid, dogfish) 

 the developing embryo absorbs much of what it needs of water, 

 salts, and probably trace elements, from seawater. Mammalian 

 embryos develop a sucker-like attachment (the placenta), and live 

 and grow practically as parasites on the wall of the mother's 

 uterus. 



The first mammalian egg observed was that of a dog, by K. E. 

 von Baer in 1827. 3 Mammalian eggs are almost microscopic, rang- 

 ing from about 70 to 85 microns in rodents (rats, mice, guinea 

 pigs) to about 140 microns in horses, dogs, sheep, goats, pigs, 

 monkeys, apes, man, and whales. Yet this tiny spherical unit, say 

 one-tenth of a millimeter in diameter, contains determinants 

 which enable it regularly to develop, under suitable conditions, 

 into its predestined individual. How is this miracle so regularly 

 accomplished? 



As an approach to this problem, let us first estimate what the tiny 

 zygote means in terms of molecules, figuring molecular dimensions 

 to be 0.2 to 5.0 millimicrons, and the zygote as a sphere whose diameter 

 is 100,000 millimicrons. The volume of this sphere (v = ^T r r s ) is 

 approximately 262 xlO 9 m/x 3 , that is, 262 million million cubic milli- 

 microns. Since a considerable percentage of the molecules are small 

 ones (water, salts) there is ample room, in addition to the chromosomes, 

 enzymes and other inclusions, for many million million highly specific 

 organic molecules supplied by the mother and the father through the 

 ovum and the sperm. Therefore there is plenty of space in the zygote 

 for an enormous number and variety of catalysts, and of specific mole- 

 cules to serve as modifiers, prosthetic groups and carriers to form, as 

 cell duplication proceeds and as new substances are produced, new 

 catalysts with new chemical ambits. 



To envisage what we desire to explain by the catalytic direction 

 of chemical change, consider as one example the following dia- 



