442 SOME GENERAL PROBLEMS OF DEVELOPMENT 



are not normal may be introduced for experimental purposes. 

 For example, the exact pattern of the cells on the outer surface of 

 the sixteen or thirty-two cell stage of a frog's egg is fundamentally 

 dependent upon the internal factors of cell division and organiza- 

 tion, but is influenced to some extent by the mechanical pressure 

 that is exerted by the cells upon one another. Each cell tends, 

 when isolated, to become a sphere, as does a single soap bubble, 

 but in the mass the outline of any particular cell is the result of its 

 relationships with neighboring cells, that is, with its environment. 

 Under normal conditions the egg of a frog or a starfish develops as a 

 spherical mass, but under the pressure of a cover slip it may be 

 caused to develop into a plate of cells. If the pressure is too long 

 continued a hopelessly abnormal embryo is produced, but if the 

 slip is removed in time the mass will gradually become adjusted 

 and a normal embryo may be formed. Again, it was thought 

 from earlier experiments, in which one of the cells in the two-cell 

 stage of the frog was killed by destroying the nucleus with a 

 needle, that the remaining cell could produce only one-half of the 

 adult, as it appears to do under normal conditions. When, how- 

 ever, a method of separating the two cells was devised, it was 

 found that each might become spherical and develop into an 

 embryo of half the normal size, as do the cells in the two-cell 

 stage of the starfish, which may be easily separated by shaking. 

 It therefore appears that one of the cells of the two-cell stage in the 

 frog produces the right side of the adult organism, and the other the 

 left side, because these cells have the inherent capacity to produce 

 a frog and not a toad, and also because each cell develops in con- 

 tact with the other. The resulting embryo is, therefore, the prod- 

 uct of a combination of hereditary and environmental factors, 

 if we consider contact with the other half as part of the environ- 

 ment for each half of the embryo. 



The influence of temperature upon the rate of development of 

 organisms is a familiar fact, as with the frog, where development 

 proceeds very slowly if the weather is cold, and more rapidly in 

 warmer weather or when the eggs are brought to the laboratory. 

 Extremes of temperature produce abnormalities, but there are 

 wide Umits. Ordinarily the temperature influences only the rate 

 and not the nature of the development and may thus be inter- 

 preted as acting through its effect upon the rate of chemical changes 

 in the embryo. Another effect of an alteration of temperature is 



