188 CHAHLES E. STOCKARD 



for the double individuals and twins among the chick embryos. 

 It also accounts most satisfactorily for the apparent frequency 

 of such occurrences. The interruption of development following 

 a fall in temperature at laying and before gastrulation has begun 

 prevents the single gastrulation process from beginning at a rate 

 sufficient to dominate the growth conditions of the entire blasto- 

 derm as it normally does. A second gastrular infolding or blas- 

 topore formation is established and thus two embryo formations 

 are begun. 



The usual interruption in the development of the bird has, 

 with slight variations, been introduced at a most fortunately pas- 

 sive stage, just following gastrulation. This is a moment at 

 which developing fish eggs may be stopped with impunity for 

 considerable lengths of time and injurious results rarely ever 

 follow. It is a moment following which no important embryonic 

 structure need arise for a considerable length of time. After 

 gastrulation only the linear growth to establish the embryonic 

 axis immediately occurs. None of the highly energetic folding 

 processes resulting from a localized excessive or unequally rapid 

 proliferation take place until after a considerable interval of slow 

 growth has passed. This interval of slow cellular proliferation 

 following gastrulation is the fortunate occurrence that has pre- 

 served the birds as a class among present-day vertebrates. Had 

 birds been so constructed that the egg was laid and allowed to 

 discontinue its development before gastrulation had taken place 

 it is conceivable that this condition could have eliminated them 

 from the animal kingdom. There would have followed such a 

 high proportion of deformed and defective specimens from eggs 

 interrupted before gastrulation, that the individuals of a class 

 having its eggs stopped at this time would very soon become so 

 generally deformed as to be unable to maintain their existence. 

 The important matter of a few hours' difference in egg-laying 

 time lies between the successful class of birds and a hopelessly 

 unfit monstrous condition. 



Obviously, the evolution of the developmental environment 

 has been of equally as great importance in the survival of a spe- 

 cies, as has been its constant structural fitness. Nature's experi- 



