32 VARIATION. 



Biomechanics was founded before Mendel's work was redis- 

 covered, before it was known, that it would ever be possible to 

 study the inherited developmental factors. For this reason, 

 nearly all the work done by the founders of Biomechanics, has 

 consisted in a study of the non-inherited, the environmental 

 factors in the development of plants and animals. It has been 

 abundantly illustrated that some of these factors may have a 

 very definite action on certain stages of the development. 

 Loeb found that a fertilized egg of a sea urchin, immediately 

 after it had produced its fertilization-membrane, formed a 

 second thin, close-fitting membrane, and that one of the 

 things necessary for the production of this membrane was the 

 presence of calcium-salts in the sea-water. If calcium was lacking 

 this membrane was not produced. Normally, the two, or four, 

 or eight cells of the young embryo are kept together by this 

 membrane. If it is not produced, the two first blastomeres tend 

 to drift apart, and each assume a spherical form. If the em- 

 bryos are brought back into normal sea-water containing cal- 

 cium, the membrane is formed, but as the two blastomeres are 

 not flattened against each other, each of the two forms its own 

 membrane, and further develops into a separate embryo. In 

 this way, twins may be produced from normal eggs in nearly 

 every instance. This case beautifully illustrates how a develop- 

 mental factor, by influencing the development at a given in- 

 stance, helps to determine the final qualities of the resulting 

 organism. 



As a typical instance of a developmental factor of another 

 class, the ripening of corn-seeds may be described. There are, 

 among other things, two kinds of corn which differ in that the 

 seeds of one species are full of starch, whereas in the other in- 

 stead of starch the seeds contain an abundance of sugar. If we 

 plant the seeds of these two species side by side, and we com- 

 pare the development of the seeds on two plants of different 

 strains we see the following result. When the seeds are grown 

 to their full size, the seeds of both plants are still soft and milky 

 and they are full of sugar. If we cut off an ear of a plant of each 



