LIMITING FACTORS IN THE HISTORY OF SCIENCE 



"All my life," he said, "I have loved and still love both 

 intellectual and manual work, and the second perhaps even more 

 than the first. Especially have I felt satisfaction when into the 

 latter I have been able to carry some good problem, thus uniting 

 head and hands. Upon this same road you are travelling. With 

 all my heart I wish that you may advance far along this path, 

 the only path that secures happiness for man."^ 



Experiment in Embryology. 



When I once gave some lectures on this subject at University 

 College, London, they bore the title "Speculation, Observation and 

 Experiment as Illustrated by the History of Em.bryology." Of the 

 first two of these factors we have seen enough, but the third would 

 necessitate the continuation of the story down to the end of the 

 nineteenth century. The true science of experimental embryology 

 did not come into being until the time of Wilhelm Roux.^ The early 

 chemical observations on the embryonic liquors were indeed observa- 

 tions rather than experiments; and there was no systematic study of 

 the changes which the liquors undergo during the development of 

 the foetus; this was not done till the time of John Dzondi.^ Harvey's 

 segregation of does at Hampton Court merits, no doubt, the name of 

 experiment, involving as it did, the use of "controls," and an out- 

 standing instance is the ligature of Nuck* in 1691. The work of Nuck 

 is very important, as one of the earliest instances of experimental 

 procedure. He ligatured the uterine horns after copulation in a dog, 

 and observed pregnancy afterwards, implantation having taken 

 place above the ligature. His conclusion was that the embryo was 

 derived from the ovary and not from the sperm — ''animal ex ovo 

 generari experimento probaturT 



As in Nuck's case, experiment in the hands of both Spallanzani^ 

 and J. T. Needham^ led to error. Spallanzani confuted his adversary 

 on the question of spontaneous generation and the vegetative force 

 by what amounted to rigid criticism of experimental conditions, but 

 later on denied their proper function to the spermatozoa on exactly 

 the same methodologically faulty grounds. 



1 Quoted in Lectures on Conditioned Reflexes, vol. II, p. 53 (London, 1941). 



'^ Gesammelte Ahhandlungen u. Entwicklungsmechanik, 1895. 



^ Journ. f. Chem. u. Physik, 1806, 2, 652. 



* Adenographia curiosa, 1691. ^ Saggi di osservaiioni microscopiche, etc., 1766. 



^ New Microscopical Discoveries, 1745. 



