146 BIOLOGICAL LECTURES. 



this question, and not a synthetic one, and must trace each 

 phenomenon down to its ultimate causes before we can begin 

 to build up dynamical hypotheses. To start the building up 

 now is a " Looking Backwards "; it is building the superstruc- 

 ture before our foundations are ready. Let us begin with our 

 foundation, let us build slowly and firmly, and the time will 

 come when we can erect our superstructure in the full assur- 

 ance that it will stand secure. 



Since the above pages were written I have had an opportunity 

 of reading an interesting paper by His,i in which he discusses 

 the application of mechanical forces to an explanation of the 

 form assumed by vertebrate embryos. He demonstrates exper- 

 imentally the conditions which govern the formation of folds, 

 such as that formed at the posterior edge of the blastoderm of 

 a shark's ovum at the beginning of the concrescence phenome- 

 non, and which represents the first formation of the medullary 

 groove, or those which we term the head and tail folds, and 

 which separate the embryo from the yolk anteriorly and posteri- 

 orly. Given the proper conditions, these folds may be explained 

 on mechanical principles, but it is the development of these 

 necessary antecedent conditions that is the important point. 

 There must be differences in the thickness of various portions 

 of the blastodermic area. There must be a greater accumula- 

 tion or a greater compactness of the cells at one portion than 

 at another, and these variations cannot at present be explained 

 on purely physical bases. The thickening of the embryonic 

 region of the blastoderm and of the germ-ring can only be 

 regarded as a preparation for what is later to develop, and the 

 cell migrations and the formation and dissolution of cell-layers, 

 which His describes, allow no mechanical explanation at present. 



His takes, it seems to me, the correct position when he states 

 " so viel ist sicher, dass sich bei der Bildung der Keimschichten 

 vitale Processe mit rein mechanischen combinieren, und dass es 

 auch hier nicht angeht, mit einigen schablonenhaften Vorstel- 

 lungen der vorhandenen Probleme Herr zu werden." The 



1 W. His : Ueber mechanische Grundvorgange thierischer Formenbildung. 

 Archiv fur Anat. u. Phys. Anat. Abtk., 1S94. 



