SECT. 3] AND EIGHTEENTH CENTURIES 213 



he could not see it. Haller's explanation of Wolff's results was that 

 the blood-vessels had been there all the time but that they had not 

 become visible until the moment at which Wolff saw the islands 

 forming. "After I had written the above", said Haller, "M. Wolff 

 made new objections against the demonstration. Instructed by new 

 researches, he denies absolutely that the yolk-membranes, which he 

 makes two in number, exist before incubation. He pretends that they 

 are new and that they are born at the beginning of incubation, and 

 consequently that the continuity of their vessels with the embryo 

 does not in the least prove that in the body of the mother the yolk 

 received vessels from the foetus. I have compared the observations 

 of this great man with my own and I have found that the yolk 

 never has more than one pulpy and soft membrane, part of which 

 is what I have called the umbilical area, and that the fine exterior 

 membrane does not belong to the yolk but to the inner part of the 

 umbilical membrane. ... I do not believe that any new vessels arise 

 at all, but that the blood which enters them makes them more 

 obvious because of the colour which it gives them, and so by the 

 augmentation of their volume, they become longer." 



Wolff replied by another extensive piece of work, which he called 

 De Formatione Intestinorum, and which appeared in one of the publica- 

 tions of the Russian Academy for 1768. It ruined preformationism. 

 In it he demonstrated that the intestine is formed in the chick by 

 the folding back of a sheet of tissue which is detached from the ventral 

 surface of the embryo, and that the folds produce a gutter which in 

 course of time transforms itself into a closed tube. The intestine, 

 therefore, could not possibly be said to be preformed, and from this as 

 starting-point, Wolff went on to propose an epigenetic theory which 

 applied the same process to all organs. It is interesting to note that 

 the facts brought forward by Wolff have never been contradicted, but 

 have been used as a foundation to which numberless morphological 

 embryologists have added facts discovered by themselves. It is 

 noteworthy that, although Wolff's second general principle, that of 

 increasing solidification during embryonic development, led to no 

 immediate results, it has been abundantiy confirmed since then (see 

 Fig. 221). His observations on the derivation of the parts of the early 

 embryo from "leaf-like" layers were even more important, and acteA 

 as a very potent influence in the work of Pander and von Baer. 



It happened, however, that Haller had much the greater in- 



