48 FRANKLIN P. REAGAN 



sac; he also observed the same process "in the embryonic body, 

 but here the process was a re-arrangement rather than an exten- 

 sive migration. So thorough were the observations of Wencke- 

 bach that hxter observations on the living telost yolk-sac by 

 Raffeale (50), Stockard (66), and myself (52, 53) have added 

 little of importance to our knowledge of the process given us by 

 Wenckebach. 



III. THE EXPERIMENTAL WORK 



The great problem of development is to determine as far as possible, 

 the sequence and origin of differentiations and the extrinsic and intrinsic 

 forces by which they may be modified. Much has already been accom- 

 plished in tracing the sequence of morphological differentiations to 

 earlier and earlier stages of development, but relatively little has been 

 learned as to the nature and causes of differentiation itself. By the 

 observation of purely normal processes one sooner or later comes to 

 a place beyond which he can make little if any progress in the study 

 of such problems; but the history of biology in the last twenty-five 

 years has shown that much may be learned by the comparison of 

 normal- and abnormal processes, and especially by the combination of 

 observations of normal processes with experiments which may be varied 

 indefinitely. — ConkHn.^ 



Prior to the year 1913 it might well have been said that the 

 status of the controversy over the origin of endothelium was 

 that in which the observation of the normal process had practi- 

 cally reached its limits, so far as the general acceptance of one 

 view or the other was concerned. Those who believed serial 

 sections to be adequate were not convinced by those who believed 

 in the adequacy of injection, and vice versa. Also, those who 

 had seen the sprouting of living endothelium failed to take into 

 account the evidence at hand that isolated mesenchyme cells 

 had been seen to form endothelium. The question resolved 

 itself into one of methods; that certain definite results could be 

 obtained by each of these diverse methods was no longer ques- 

 tioned by any one. As affirmed by Bartels concerning one of 

 the methods, we might well affirm concerning the questions 

 involved in all the methods employed; they were '' philosophical, 

 and not anatomical questions." The account of the develop- 



1 Journal of the Academy of Natural Sciences of Philadelphia, vol. 15, Second 

 Series, 1912, pp. 503-4. 



