118 CHARLES R. STOCKARD 



moment will produce a double monster or identical twins and at 

 another moment slowing by the same method will give rise to 

 the Cyclopean defect. In fact, the same thing which causes 

 the double monster may later in development induce one of its 

 heads to be cyclopean. 



Thus there is no longer any ground for considering certain 

 defects as specific responses to particular treatments. And there 

 is as little reason for further descriptions of individual monsters, 

 since all belong to the same class and the individual differences 

 simply result from the different moments during which the devel- 

 opmental interruptions have acted. 



The important consideration then arises as to what internal 

 and external factors may tend to introduce the developmental 

 arrests. Does one growing part in any way inhibit the activity 

 of other developing organs? We shall devote a section to a con- 

 sideration of the interaction among the developing and growing 

 organs within the embryo. The study of the growth influences of 

 one embryonic . organ on another is one of the most important 

 problems in the analysis of structure. 



Finally, the interaction among growing parts and the inhibit- 

 ing effects of one rapidly proliferating region over other regions 

 will be very briefly considered in connection with abnormal and 

 malignant growths. 



2. THE specific' RATE OF DEVELOPMENT IN A GIVEN SPECIES 



It is a generally known fact that the eggs of different species 

 do not progress at the same rate of development even during 

 comparable stages. The lengths of time between fertilization 

 and the first cleavage and the rates at which the early cleavages 

 follow one another may differ decidedly among the eggs of 

 even closely related forms. These differences in developmental 

 rate are probably fundamentally connected with differences in 

 chemical structure of the egg substances, and in particular with 

 the different rates of oxidation of certain stuffs. It is a well- 

 known chemical fact that very slight differences in composition 

 between substances may cause very great differences in their 

 oxidation capacities. 



