124 How Living Things Come to be What They Are 



The inscription, omne vivo ex ovo — all life from the Q,g% — 

 was neither original with Harvey nor of the same significance 

 as that which later biologists attached to the expression. In 

 Harvey's mind, the ** q%% " was simply the germ or beginning. 

 Today the biologist would go farther and say that every 

 individual starts out upon life as a single cell. 



In the simplest plants and animals the individual begins 

 life as a single cell and never gets to be any more than that. 

 In more complex animals and plants, the simple living mass 

 divides itself into two cells, which adhere to each other and 

 again divide, making four cells. Each of the four divides 

 making eight, and so on. This process of cleavage or succes- 



Fig. 34. Early Stages in the Development of a Frog 



In the frog's egg there is a considerable amount of food matter, or yolk, in addition to 

 the protoplasm. This yolk material is heavier than the protoplasm, and remains at the 

 bottom of the mass. So long as the cell divisions are in a vertical plane, b, c, all the cells 

 formed may be alike; but when walls are formed in a horizontal plane, d, the upper cells 

 will be smaller than the lower ones, for while all the cells may have the same amount of 

 protoplasm, the lower ones will have larger quantities of yolk and will thus be larger, 

 e, f, g. From Gruenberg, Biology and Human Life, published by Ginn & Company. 



sive cell division has been observed in every species of animal 

 and of plant so far studied. After the first few cell divisions 

 the further development proceeds along a variety of paths. 

 If we follow a single frog's egg we may see the formation 

 of a ball consisting of several cells. The ones near the top 

 of the sphere are smaller than the lower ones. Although the 

 first four cells were indistinguishable there gradually appears 

 a differentiation. In some parts of the mass the cells increase 

 in numbers more rapidly (see Fig. 34) . If we follow another 

 frog's egg we shall find exactly the same procedure up to 

 the point where a tadpole is clearly discernible, and a third 



