EXPERIMENTAL EMBRYOLOGY. 

 By E. A. Andrews. 



The accumulation of embryological facts and their applica- 

 tion to problems of animal morphology from the days of von 

 Baer to the period of Balfour's text book of Comparative 

 Embryology was carried on with ever increasing speed 

 culminating in the present day when the revision of Balfour's 

 work by Korschelt and Heider assumes such unexpected pro- 

 portions. Though the advance of descriptive emhryology has 

 been so great, the phy>iologieal aspects of the subject have 

 been but little cultivated, partly to be sure, from the neeessa in- 

 dependence of such work upon the anatomical fact- that 

 were not at first available. Now, however, when the normal 

 development is known for all groups of animals and com- 

 parative embryology stands upon a firm basis, the application of 

 physiological methods, the introduction of experimentation 

 into a field promising much richer harvest than the study of 

 adults can hope to yield, may be no longer delayed. Knowing 

 the changes of form that ova pass through to attain the 

 adult condition may we not both eliminate such changes as 

 are unessential and also press nearer to the solution of more 

 fundamental questions by varying the condition of environ- 

 ment and the physical state of the ovum or embryo ? 



Interference with the normal course of embryological 

 phenomena was no doubt often brought about more or less 

 unconsciously, or at least incidentally, by many of the older 

 embryologists and remarkable results sometimes attained. 

 Only, however, within the present decade have systematic 

 researches been begun, definite and thought-out experiments 

 devised and finally predictable results attained by workers in 

 the domain of what may be called experimental embryology, 

 though as yet the methods and the subject matter are so 

 differently conceived by various authors and the question 

 involved so overlaps the regions assigned to other branches of 

 Biology that the term has at best but a vague and changing 



