STRUCTURE AND DEVELOPMENTAL RATE 189 



merit of temporarily lowering the surrounding temperature and 

 stopping the developmental progress of the bird's egg has not 

 proved fatal simply on account of the fortunate fact that the 

 development is usually stopped during a very passive stage. 



The slight individual variations in egg-laying time which cause 

 certain eggs to be interrupted before gastrulation very probably 

 furnish the material for the many descriptive studies of double 

 avian embryos. On the other hand, it is a most significant fact 

 to note that in spite of the many experimental studies on devel- 

 oping hen's eggs by Dareste, Fere, the writer, and others no 

 double monster or twin conditions have been produced. ' This 

 absence of double productions would naturally be expected, 

 since the eggs were experimentally treated only after having been 

 laid. They had thus passed gastrulation or the time after which 

 double conditions cannot be induced. 



Gerlach ('82) long ago thought that he had probably induced 

 experimentally double anterior ends in chick embryos. His re- 

 sults were most uncertain, and have been interpreted as acci- 

 dental by subsequent writers. He made injections over the bias- 

 derm so as to get fusions with the overlying shell. With such 

 experiments he obtained double indications at the forward end 

 of the embryos in two cases out of sixty eggs. Gerlach realized 

 that conclusions could not be drawn from these meager results, 

 but believed that if this method were perfected, it would yield 

 more convincing results. Such experimental efforts to produce 

 doubled conditions in hen's eggs are very probably futile, since 

 the evidence at hand would indicate that there is only the rarest 

 chance of the experimenter's striking an egg in the proper devel- 

 opmental condition to make possible the production of twin or 

 double individuals. Should such specimens be obtained among 

 the eggs employed in an experiment, there would always be the 

 possibility that the natural interruption in development occur- 

 ring in an egg laid at an unusually early stage was the cause of 

 the doubleness, and not actually the experimental procedure. 



