212 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii 



Wolff's Theoria generationis, which was a defence of epigenesis on 

 theoretical and philosophical grounds, written in a very formal, 

 logical, and unreadable manner, appeared when he was only twenty- 

 six years old, in 1759. Leibnitz, as Radl points out, had borrowed 

 from the earlier preformationists the conception of a unit increasing 

 in bulk in order to become another kind of unit; but Wolff, following 

 Needham, borrowed from Leibnitz the idea of a monad developing 

 into an organism by means of its own inherent force, and to this he 

 joined the Stahlian notion of a generative supra-physical force in 

 nature. On the practical side, Wolff's work was indeed of the highest 

 importance. If the embryo pre-exists, he argued, if all the organs 

 are actually present at the very earliest stages and only invisible to 

 us even with the highest powers of our microscopes, then we ought 

 to see them fully formed, as soon as we see them at all. In other 

 words, at the moment at which any given organ comes into view, it 

 ought to have the form and shape, though not the size, of the same 

 organ when fully completed in the embryo at birth. On the other 

 hand, if this is not the way in which development goes on, then one 

 ought to be able to see with the microscope one shape changing into 

 another shape, and, in fact, a series of appearances, each one different 

 from that which had immediately preceded it, or, in other words, 

 a series of advancing adaptations of the various parts of the primitive 

 embryonic mass. WoliT chose as his first test case the blood-vessels 

 of the blastoderm in the chick, for he saw that at one moment this 

 apparatus was in existence, while the moment before it had not been. 

 His microscopical researches led him to the conclusion that the homo- 

 geneous surface of the blastoderm partially liquefies and transforms 

 itself at these points into a mass of islands of solid matter, separated by 

 empty spaces filled with a colourless liquid but afterwards with a red 

 liquid, the blood. Finally, these spaces are covered with membranes 

 and become vessels. Consequently it was obvious that the vessels had 

 not been previously formed, but had arisen by epigenesis. 



Haller replied to this new experimental foundation for epigenesis 

 without delay, for he was working on the development of the chick 

 at the same time, and held closely to the opposite theory. We have 

 already seen what his one and only argument against Wolff was. He 

 used it time after time in all its possible variations, maintaining stoutly 

 that the chick embryo was so fluid in the early stages that Wolff had 

 no right to deny the presence of a given structure simply because 



