18 LIBBIE H. HYMAN. 



rate will be completely suppressed or will be abnormal because the 

 rate of activity necessary for their normal development cannot be 

 attained in such depressed eggs or embryos. The various agents 

 which can be used to affect development are regarded from this 

 viewpoint as acting merely in a quantitative way to retard or 

 accelerate development; and this action is differential affecting 

 some parts of the embryo more than others since some parts have 

 or require a higher rate of activity than other parts. 



Interpretations of teratological development somewhat similar 

 to the foregoing have not been wanting. Dareste ('91) who 

 investigated more thoroughly than anyone else before or since 

 the experimental production of monstrosities in the chick ex- 

 pressed similar ideas. He states that development is due in 

 part to inherent tendencies and in part to external conditions. 

 As the latter can be altered, modification of the course of develop- 

 ment is possible. Dareste reached the following important 

 general conclusions, (i) The same abnormalities are produced 

 by very different conditions, there being then no necessary 

 relation between the application of a certain condition and the 

 appearance of certain modifications. (2) Embryos submitted to 

 the action of identical factors do not necessarily present the same 

 abnormalities (although these may often be similar). The reason 

 for this is that eggs are inherently different from each other in a 

 variety of ways. (3) The different abnormalities do not depend 

 on the nature itself of the teratogenic agents but on the time 

 at which they act upon the embryo, their intensity, and their 

 duration. (4) All abnormalities consist essentially of an arrest 

 of development of the embryo or its membranes. The organs 

 of an embryo appear successively; they pass through a certain 

 number of stages. An arrest of development consists in the 

 persistence of an embryonic state ordinarily transitory. Among 

 the abnormalities assigned by Dareste to arrest of development 

 at a certain stage are: duality of the heart, spina bifida, absence 

 or poor development of the area vasculosa, absence or ab- 

 normalities of the amnion and allantois, defects of the brain, head, 

 spinal cord, and sense organs. Certain subsequent interpreta- 

 tions of teratological development as for instance that of Stockard 

 ('21) do not appear to differ from or add anything to Dareste's 

 conclusions. 



