Ill] THE DOCTRINE OF PREFORMATION 55 



them, and which are characteristic of another order of magni- 

 tude. 



By other writers besides Haller the very general, though not 

 strictly universal connection between form and rate of growth 

 has been clearly recognised. Such a connection is implicit in 

 those "proportional diagrams" by which Diirer and some of his 

 brother artists were wont to illustrate the successive changes of 

 form, or of relative dimensions, which attend the growth of the 

 child, to boyhood and to manhood. The same connection was 

 recognised, more explicitly, by some of the older embryologists, 

 for instance by Pander*, and appears, as a survival of the 

 doctrine of preformation, in his study of the development of 

 the chick. And long afterwards, the embryological aspect of 

 the case was emphasised by His, who pointed out, for instance, 

 that the various foldings of the blastoderm, by which the neural 

 and amniotic folds were brought into being, were essentially 

 and obviously the resultant of unequal rates of growth, — of 

 local accelerations or retardations of growth,^ — in what to begin 

 with was an even and uniform layer of embryonic tissue. If 

 we imagine a flat sheet of paper, parts of which are caused 

 (as by moisture or evaporation) to expand or to contract, the 

 plane surface is at once dimpled, or "buckled," or folded, by 

 the resultant forces of expansion or contraction : and the various 

 distortions to which the plane surface of the "germinal disc" is 

 subject, as His shewed once and for all, are precisely analogous. 

 An experimental demonstration still more closely comparable to 

 the actual case of the blastoderm, is obtained by making an 

 "artificial blastoderm," of httle pills or pellets of dough, which 

 are caused to grow, with varying velocities, by the addition 

 of varying quantities of yeast. Here, as Roux is careful to 

 point outt, we observe that it is not only the growth of the 

 individual cells, but the traction exercised through their mutual 

 interconnections, which brings about the foldings and other dis- 

 tortions of the entire structure. 



* Beitrdge zur Entwickelungsgeschichte des Hiihnchens im Ei, p. 40, 1817. Roux 

 ascribes the same views also to Von Baer and to R. H. Lotze (Allg. Physiologie, 

 p. 353, 1851). 



t Roux, Die Entwickdungsmeclianik, p. 99, 1905. 



