STRUCTURE AND DEVELOPMENTAL RATE 123 



Changing the temperature of the environment and, therefore, 

 of the egg, is the method employed in most of the present experi- 

 ments in order to interrupt or make discontinuous a normally 

 continuous development. 



There are several definite natural cases of discontinuous devel- 

 opment among mammals, the significance of which will be con- 

 sidered in another section of this paper. But in the present con- 

 nection we may be certain that nature has here employed 

 another method than temperature change in causing the interrup- 

 tion. The temperature of the maternal body in which the mam- 

 malian embryo is developing is sufficiently uniform never to 

 interrupt the progress of the egg. For reasons to be more fully 

 cited beyond, changes in the supply of oxygen would seem to be 

 the most probable cause of interrupted development in the rare 

 cases of this phenomenon among mammals. Lack of oxygen or 

 excess of CO2 has also been resorted to in the present experiments 

 as a means of interrupting or retarding the rate of a normally 

 continuous development. 



Neither of the two methods is new. A number of experi- 

 menters have studied the influence of temperature changes on 

 the manner of development of different eggs. The effects of 

 abnormally high and low incubator temperature on the develop- 

 ment of the hen's egg have been recorded by Dareste and many 

 others, most recently by Miss Alsop ('19). The development 

 of amphibian eggs under unusual temperature conditions has 

 been considered by O. Hertwig ('96), King ('04), and others. 

 The influences of low temperatures on the development of the 

 fish's egg have been investigated by Loeb ('16) and Kellicott 

 C16). 



These studies on temperature, however, are of interest in the 

 present connection only in so far as they almost all show how 

 readily abnormal development of the embryo may be induced by 

 unfavorable temperature conditions. The attempted explana- 

 tions of the deformities which were given in only a few cases, as 

 by Kellicott, entirely disregard or dismiss the real point of fun- 

 damental importance; that is, the induced change in the rate of 

 development resulting from the modified temperature. Kellicott 



