EMBRYOLOGICAL CRITERION OF HOMOLOGY. 119 



by which the body of the embryo vertebrate is formed are pro- 

 duced by definite distribution of horizontal growth in the 

 blastoderm in connection with mechanical limitations which 

 oppose its lateral extension. 



A conception essentially similar to this, though elaborated 

 with far greater detail and subtlety, lies at the foundation of 

 Driesch's and Hertwig's theories of development ; and similar 

 views appear in the writings of Nageli, Vbchting, and many 

 others. In Driesch's writings, which in some respects appear 

 to express most adequately the present aspect of the problem 

 (though few, probably, will accept his more extreme views), we 

 find every step in development regarded as a physiological reac- 

 tion of the idioplasm (i.e., an "Auslosung ") to the conditions 

 of its environment, internal or external, existing at the time. 

 The nature of the reaction (i.e., of any particular phenomenon 

 in the development) depends always upon the two factors 

 recognized by His, viz., primarily upon the nature of the idio- 

 plasm, which is predetermined, and secondarily upon the condi- 

 tions existing at the moment of the reaction. These conditions 

 are in part external to the embryo (chemical composition of 

 the surrounding medium, temperature, pressure of membranes, 

 osmotic relations, and the like), in part internal (effects of 

 food-yolk, surface-tensions, chemical differences, mutual pres- 

 sure of cells, and no doubt a multitude of unknown physiological 

 relations between the developing parts), the latter being pro- 

 gressively created by the activity of the idioplasm itself. 1 



We may now return from this digression to a consideration 

 of the question of homology. It is clear, according to the 

 view just outlined, that development may be altered in two 

 ways, viz., either by congenital changes in the idioplasm (which 

 must in a greater or less degree involve correlated changes in 

 the internal conditions), or by changes in the external condi- 

 tions under which the idioplasm operates ; and it is this fact 

 that renders it so difficult, in the present state of our knowl- 



1 Hertwig's Zeit- und Streitfragen der Biologic (received during the preparation 

 of the MS. of this lecture) gives an extremely clear and suggestive discussion of 

 a theory of development similar in many respects to that here outlined. 



