STEUCTTJRE AND DEVELOPMENTAL RATE 119 



The efforts on the part of numerous embryologists to associate 

 the differences in rate of cleavage and time required to attain 

 certain stages of development with the size of the egg, the amount 

 and position of the yolk substances, or even the types of cleavage 

 have not been satisfactory. Certain meroblastic eggs develop 

 much faster than certain holoblastic ones, while other holoblas- 

 tic eggs have a rate of cleavage far more rapid than the mero- 

 blastic types. All of the so-called laws of cleavage rates based 

 on morphological differences among egg types have been found 

 to fail so decidedly when applied in general that one is forced 

 to seek more deep-seated causes for the differences in develop- 

 mental rate. 



At the present time we can only state that such causes probably , 

 reside in the differences in chemical make-up of the several 

 species of eggs. The rate of development certainly depends, 

 particularly during later stages, on the amount of food avail- 

 able, but the supply of oxygen and the degree of temperature at \ 

 which development is taking place have a far more striking influ- \ 

 ence on the rate. Cessation of development also occurs much \ 

 more promptly from absence of oxygen or sudden changes in 

 temperature than from any other natural modifications which 

 happen in the environment. These facts point decidedly to the 

 rate of development as being dependent upon kind and rate of 

 chemical change, most particularly upon rate of oxidation. The 

 egg probably has a definite coefficient of metabohsm dependent 

 upon the interaction of its specific cheiniCaT^tructure and the 

 given environment in which it normally develops. The rate of 

 development results from both the internal qualities of the egg 

 and the nature of the surrounding environment. 



The present extremely crude state of our knowledge of the 

 chemistry of development will permit of no more satisfactory 

 statements of the principles underlying differences in develop- 

 mental rate than those which have been attempted above. The 

 inadequacy of such statements is as keenly appreciated by the 

 writer as by the critical reader, but this inadequacy concerns 

 chiefly, the absence of the details involved, while the statements 

 in general I believe are correct. 



