84 THE RATE OF GROWTH [ch. 



certain Nostoc-algae in which unequal growth, ceasing towards the 

 periphery of a disc and increasing here and there within, gives rise 

 to folds and bucklings curiously Hke those of our own ears: which 

 indeed owe their shape and characteristic folding to an identical or 

 analogous cause. 



An experimental demonstration comparable to the actual case is 

 obtained by making an "artificial blastoderm" of Uttle pills or 

 pellets of dough, which are caused to grow at varying rates by the 

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

 to point out,* it is not only the growth of the individual cells, but 

 the traction exercised on one another through their mutual inter- 

 connections, which brings about foldings, wrinklings and other 

 distortions of the structure. But this again, or such as this, had been 

 in Haller's mind, and formed an essential part of his embry illogical 

 doctrine. For he has no sooner treated of incrementum, or celeritas 

 incrementi, than he proceeds to deal with the contributory and 

 complementary phenomena of expansion, traction (adtractio)^ and 

 pressure, and the more subtle influences which he denominates vis 

 derivationis et revulsionis%'. these latter being the secondary and 

 correlated effects on growth in one part, brought about by such 

 changes as are produced, for instance in the circulation, by the 

 growth of another. 



We have to do with growth, with exquisitely graded or balanced 

 growth, and with forces subtly exerted by one growing part upon 

 another, in so wonderful a piece of work as the development of the 

 eye : as its primary vesicle expands and then dimples in, as the lens 

 appears and fits into place, as the secondary vesicle closes over to 

 form iris and pupil, and in all the rest of the story. 



Let us admit that, on the physiological side, Haller's or His's 

 methods of explanation carry us but a little way; yet even this 

 little way is something gained. Nevertheless, I can well remember 



* Roux, Die Entwickelungsmechanik, 1905, p. 99. 



t Op. cit. p. 302, "Magnum hoc naturae instrumentum, etiam in corpore animato 

 evolvendo potenter operatur, etc." The recurrent laryngeal nerve, drawn down 

 as its arch of the aorta descends, is a simple instance of anatomical traction. The 

 vitelline and omphalomesenteric arteries lead, by more complicated constraints 

 and tractions, to the characteristic loops of the intestinal blood vessels, and of the 

 intestine itself. Cf. G. Enbom, Lunds Univ. Arsskrift, 1939. 



X Ibid. p. 306, "Subtiliora ista, et aliquantum hypothesi mista, tamen magnam 

 mihi videntur speciem veri habere." 



