time: the refreshing river 



turning-point in the science. It may here be noted, however, that even 

 when methods are available, the workers of the time do not necessarily 

 use them, and although Harvey could have employed an early form of 

 microscope, he voluntarily restricted himself to the weak lenses, 

 "perspicilia," or perspectives, which had already been used by 

 Riolanus. A still more obvious instance is that of artificial incubation. 

 Carried on in Egypt since the remotest antiquity, this process must 

 have been at the disposal of Egyptian physicians, Alexandrian bio- 

 logists, and Arabian scholars for a period of three thousand years, 

 yet so far as we know, no embryological use of it was ever made. 

 In eighteenth-century France and England the technique of the 

 process had to be painfully rediscovered at a time when biologists 

 were only too eager to make use of such assistance. Let us mention, 

 as other instances of the effect of material technique on embryology, 

 the burst of knowledge which followed the invention of the auto- 

 matic microtome by ThrelfalF and others about 1 860, and the great 

 advance which in our own century has followed the successful 

 mastery of grafting technique by Spemann.^ 



Just as important, however, as material technique is mental tech- 

 nique. And first with respect to words; on several occasions we have 

 had to notice a standstill on account of the lack of a satisfactory 

 terminology. Thus in the thirteenth century Albertus of Cologne^ 

 had arrived at a point beyond which progress was impossible in the 

 absence of new words. When, for example, there was no other means 

 of describing the sero-amniotic junction in the hen's egg than by 

 speaking of "the hole on the left side of the vessel which runs above 

 the membrane on the right hand of something else," accuracy was 

 difficult and speed impossible. A precisely similar position was 

 occupied by Boerhaave* in the eighteenth century, only now in the 

 case of biochemical words. Faced with some substance such as a 

 "greasy, streaky yellow oil, smelling of alkaline salt" Boerhaave was 

 unable to describe it except in these common-sense terms, and lack- 

 ing the means either to submit it to further analysis or to characterise 

 it by accurate physico-chemical constants, he was forced to admit a . 

 vast number of ultimates into his schemes which were not ultimate 

 at all. 



Mental technique, as a limiting factor in embryological history, 



^ See Biol. Rev., 1930,5, 357. 



2 See my Biochemistry and Morphogenesis (Cambridge, 1942). 



3 De Animalibus^ ed. Stadler, 1916. * Elementa Chetnlae, 1732. 



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