2 PROLEGOMENA 



given of the events going on in the maternal organism during preg- 

 nancy : for the present purpose the discussion will go as far into the 

 mother as the placenta but no farther. Again, hatching or birth will 

 put an end to the discourse as to the foetal state itself, save that, in 

 the cases of animals which hatch before the yolk-sac is absorbed, 

 their embryonic life is assumed to end when they first take food for 

 themselves. Appendices are added dealing with the plant embryo 

 and the insect pupa, which, in the later stages of metamorphosis, 

 have points both of resemblance to and of difference from the growth 

 of the embryo. It is natural to hope that the outcome of all this labour 

 may be an increase of interest among biologists in this section of their 

 domain, and a great accession to the number of those investigators 

 who devote their energies to actual experiments in this new field. 



For it must be confessed that it is a new field. It has been opened 

 up in very gradual stages: fitful and sporadic experiments on the 

 constitution of embryonic tissues in the seventeenth century, a gradual 

 growth of knowledge about the chemical composition of eggs in the 

 eighteenth, a big increase of activity in the early nineteenth; d'lTxiug 

 which appear the first observations on the physico-chemical changes 

 taking place in the embryo during its development, and then in our 

 own time a mass of very widely scattered work bringing the subject 

 up to the "obstetrical" stage. Such a work as this, in my opinion, 

 should not be compared with laboratory experiments in a derogatory 

 sense, for, while it is true that facts are the ultimate court of appeal 

 in any scientific discussion, yet at the same time the number of in- 

 vestigators has grown to such extraordinary proportions in this century 

 that some danger exists lest we should be so busily engaged in accu- 

 mulating new facts as to be left with no time at all to devote any 

 thought to those we have already. Classification, indexing, and 

 maturer consideration about the facts we actually possess are at least 

 as great a need at the present moment as the invention of new facts. 

 "Everyone must realise", says Eugenio Rignano, "how much this 

 theoretical elaboration, performed by means of analyses and com- 

 parisons, of generalisations and hypotheses controlled and verified by 

 the correspondence of facts with the results of the reasoning, is useful 

 and necessary if one wishes to reach a progressive systematisation and 

 an ever more synthetic vision of the confused mass of facts which 

 experimentalists pour daily in a continuous stream into the scientific 

 market." 



