I INTRODUCTORY 17 



has very largely broken down, Epigenesis, taken by itself, 

 remains, not a theory in terms of cause and effect, but a mere 

 description of what occurs, and it is the crying need for such a 

 theory that has given birth to modern experimental embryology. 



The new era opens with the publication by Wilhelm His in 

 1874, just a hundred years after the appearance of the Theoria 

 Generationis, of a remarkable series of essays entitled Unsere 

 Korperform nncl das pkysiologische Problem Hirer Entstekung. In 

 these essays His, who was already in revolt against the ' Biogenetic 

 Law ', not only sought to give a mechanical explanation of 

 differentiation, but also laid down his famous 'Prinzip der organ- 

 bildenden Keimbezirke/ According to this principle of 'germinal 

 localization ', every spot in the blastoderm corresponds to some 

 future organ : ' das Material zur Anlage ist schon in der ebenen 

 Keimscheibe vorhanden, aber morphologisch nicht abgegliedert, 

 und somit als solches nicht ohne Weiteres erkennbar.' * Conversely, 

 every organ is represented by some region in the blastoderm, and 

 ' wenn wir consequent sein wollen ' in the fertilized, or even 

 unfertilized, egg. In other words, although the parts of the 

 embryo cannot be said to be preformed in the germ, the materials 

 for those parts are already there, prelocalized, arranged roughly, 

 at least, as the parts themselves will be later on. In this 

 material unequal growth produces the form of the parts, and so 

 of the whole body. Whether there is a strict causal connexion 

 between each material rudiment and the organ which arises from 

 it, whether these rudiments could be interchanged without 

 prejudice to the normality of subsequent development, is a ques- 

 tion which is not touched upon by His. It was reserved for 

 another anatomist, Wilhelm Roux, to raise what in His's hands 

 had been merely a principle to the rank of a theory, the ' Mosaik- 

 theorie ', or theory of self-differentiation. 



For Roux, no doubt, the f Mosaik-theorie ' was in part the 

 outcome of the theoretical necessity of explaining the specific 

 nature of development ; but it rests also upon a basis of observa- 

 tion and experiment. The coincidence in a majority of Frogs' 

 eggs of the first furrow with the sagittal plane, the production 

 of local defects in the embryo by local injuries to the egg, the 



1 His, 1. c., ii. 



JENKINSON C 



