THE THEORIA GENERATIONIS. 



273 



of his opponents. He did neither. His main contention is 

 clear enough, although not often expressly stated. He rarely 

 refers directly to the theory of predelineation and when he 

 does there is no sting in his refutation. 



Stripped of many details that are somewhat wearisome to 

 the modern reader, the result of Wolff's observations may be 

 expressed in his own words taken from the very middle of the 

 Theoria} I translate : " In general we cannot say that what 

 cannot be perceived by the senses does not therefore exist. 

 This principle is more facetious than true when applied to 

 these observations. The particles which constitute all animal 

 organs in their earliest inception are little globules, which may 

 always be distinguished under a microscope of moderate mag- 

 nification. How, then, can it be maintained that a body is 

 invisible because it is too small, when XVq parts of which it is 

 composed are easily distinguishable 1 " If we of to-day read 

 in the place of "globules" the word "cells," which are what 

 Wolff actually saw and distinguished in both plants and animals, 

 we shall have no difficulty in understanding how his observa- 

 tions disproved emdoi/ement, at least for any one who would 

 take the trouble to repeat them. Wolff had looked further 

 than the adult form and had found not a series of similar, 

 incapsulated embryos, but a single embryo made up of a vast 

 number of minute particles, the cells, closely resembling one 

 another, but placed side by side. There was no expanding of 

 a preexisting organism till it entered the field of vision, but a 

 host of minute and always visible elements that assimilated 

 food, grew and multiplied, and thus gradually in associated 

 masses produced the stem, leaves, stamens — in short, every 

 organ of the plant. This he shows in the first part of the 

 Theoria. In the second part, carrying on the same method, he 

 shows how in the animal body the heart, blood vessels, limbs, 

 alimentary canal, kidneys, etc., arise in a similar manner. 



The third part of the work is devoted to theoretical con- 

 siderations. Wolff conceives living things to be constructed 

 like a plant, to consist of a main stem, or trunk with roots and 

 branches. In the embryo of the bird the umbilical duct cor- 



1 Theoria Generationis, p. 72. 



