Ill] OF PHYSICS AND EMBRYOLOGY 85 



the harsh criticism and even contempt which His's doctrine met 

 with, not merely on the ground that it was inadequate, but because 

 such an explanation was deemed wholly inappropriate, and was 

 utterly disavowed*. Oscar Hertwig, for instance, asserted that, in 

 embryology, when we find one embryonic stage preceding another, 

 the existence of the former is, for the embryologist, an all-sufficient 

 "causal explanation" of the latter. "We consider (he says) that 

 we are studying and explaining a causal relation when we have 

 demonstrated that the gastrula arises by invagination of a blasto- 

 sphere, or the neural canal by the infolding of a cell-plate so as to 

 constitute a tubef." For Hertwig, then, as Roux remarks, the task 

 of investigating a physical mechanism in embryology — "der Ziel das 

 Wirken zu erforschen" — has no existence at all. For Balfour also, 

 as for Hertwig, the mechanical or physical aspect of organic develop- 

 ment had little or no attraction. In one notable instance, Balfour 

 himself adduced a physical, or quasi-physical, explanation of an 

 organic process, when he referred the various modes of segmentation 

 of an ovum, complete or partial, equal or unequal and so forth, to 

 the varying amount or varying distribution of food-yolk associated 

 with the germinal protoplasm of the egg. But in the main, like all 

 the other embryologists of his day, Balfour was engrossed in the 



* Cf. His, On the Principles of Animal Morphology, Proc. R.S.E. xv, 

 p. 294, 1888: "My own attempts to introduce some elementary mechanical or 

 physiolbgical conceptions into embryology have not generally been agreed to by 

 morphologists. To one it seemed ridiculous to speak of the elasticity of the germinal 

 layers; another thought that, by such considerations, we 'put the cart before 

 the horse'; and one more recent author states, that we have better things to do 

 in embryology than to discuss tensions of germinal layers and similar questions, 

 since all explanations must of necessity be of a phylogenetic nature. This opposition 

 to the application of the fundamental principles of science to embryological questions 

 would scarcely be intelligible had it not a dogmatic background. No other explana- 

 tion of living forms is allowed than heredity, and any which is founded on another 

 basis must be rejected.. . .To think that heredity will build organic beings without 

 mechanical means is a piece of unscientific mysticism." Even the school of 

 Entwickelungsmechanik showed a certain reluctance, or extreme caution, in speaking 

 of the physical forces in relation to embryology or physiology. This reluctant 

 caution is well exemplified by Martin Heidenhain, writing on "Formen und Krafte 

 in der lebendigen Natur" in Roux's Vortrdge, xxxir, 1923. Speaking of "die 

 Krafte welche die Entwickelung und den fertigen Zustand der Formen bedingen", 

 he says: "letztere kann man aber nicht auf dem Felde der Physik suchen, sondern 

 nur im Umkreis der Lebendigen, obwohl anzunehmen ist, dass diese Krafte spater 

 einmal ' analogienhaft ' nach dem Vorbilde der Physik beschreibbar sein werden" 



t 0. Hertwig, Zeit- und Streitfragen der Biologie, ii, 1897. 



