234 BIOLOGY AND ITS MAKERS 



out, and the palaeontological records arc searched, until the 

 evidence from both sources establishes the fact that the organ 

 or organism under study is but the summation of modifica- 

 tions and elaborations of a relatively simple primordial. This 

 point settled, physiology is called upon to complete the story. 

 Have the functions remained the same through the series? 

 or have they undergone a series of modifications, differentia- 

 tions, and improvements more or less parallel with the mor- 

 phological series?" 



Since physiology is an experimental science, all questions 

 of this nature must be investigated with the help of experi- 

 ments. Organisms undergoing development have been sub- 

 jected to changed conditions, and their responses to various 

 forms of stimuli have been noted. In the rise of experimental 

 embryology we have one of the most promising of the recent 

 departures from the older aspects of the subject. The results 

 already attained in this attractive and suggestive field make 

 too long a story to justify its telling in this volume. Roux, 

 Herbst, Loeb, Morgan, E. B. Wilson, and many others have 

 contributed to the growth of this new division of embryology. 

 Good reasons have been adduced for believing that qualitative 

 changes take place in the protoplasm as development pro- 

 ceeds. And a curb has been put upon that "great fault of 

 embryology, the tendency to explain any and every operation 

 of development as merely the result of inheritance." It has 

 been demonstrated that surrounding conditions have much 

 to do with individual development, and that the course of 

 events may depend largely upon stimuli coming from with- 

 out, and not exclusively on an inherited tendency. 



Cell-Lineage. — Investigations on the structural side have 

 reached a high grade of perfection in studies on cell-lineage. 

 The theoretical conclusions in the germ-layer theory are 

 based upon the assumption of identity in origin of the differ- 

 ent layers. But the lack of agreement among observers, espe- 



