rEBEUABY 24, 1905.] 



SCIENCE. 



283 



of visible structure. Its development, 

 said Harvey and Wolff, is essentially a pro- 

 cess of 'epigenesis'— a successive formation 

 and addition of new parts not previously 

 existent as such in the egg. This conclu- 

 sion, roughly outlined by Aristotle, was 

 apparently established on an irrefragable 

 basis of observation,, long afterwards, by 

 Harvey and Wolff. In its superficial as- 

 pects the doctrine of epigenesis is no more 

 than a statement of universally admitted 

 fact. When followed to its logical end, 

 however, this conception has failed, and 

 will always continue to fail, to satisfy the 

 mind ; and some of the most acute of 

 modern embryologists have expressed the 

 opinion that no thoroughgoing hypothesis 

 of epigenesis can be so framed as to be 

 logical, or even conceivable. Even in the 

 eighteenth century this doctrine was met 

 by the opposing one of preformation and 

 evolution. Advocated by such men as 

 Malpighi, Haller and Leibnitz, this con- 

 ception underwent its fullest development 

 in the hands of the eminent Swiss natural- 

 ist Bonnet. Developed with great logical 

 acuteness and set forth \v\\h captivating 

 literary skill, Bonnet's theory was based on 

 the fundamental assumption that the em- 

 bryo, though invisible, really exists pre- 

 formed in the egg before development be- 

 gins. The preformed germ was not con- 

 ceived to be an exact miniature model of 

 the adult. On the contrary. Bonnet 

 thought of the germ of the fowl, for ex- 

 ample, as differing widely in form and pro- 

 portions from an actual bird, still the 

 original preformation was assumed to be 

 composed of parts that correspond, each 

 for each, to the parts of the chick. De- 

 velopment, accordingly, was conceived to be 

 only the unfolding and transformation of 

 a preexisting structure, not the successive 

 formation of new parts— a process of 'evo- 

 lution,' not of epigenesis. In this partic- 

 ular form the doctrine of preformation was 



conclusively overthrown by Wolff; but the 

 principle underlying it has repeatedly and 

 persistently reappeared in later specula- 

 tions on development, and still contests the 

 field of discussion with its early antagonist. 



Hand in hand with this controversy has 

 gone one of still more general scope be- 

 tween the two opposing conceptions that 

 I have referred to as mechanism and vital- 

 ism. Is development at bottom a mechan- 

 ical process? Is the egg a kind of com- 

 plex machine, wound up like a piece of 

 clockwork, and does development go for- 

 ward like the action of an automaton, an 

 inevitable consequence of its mode of con- 

 struction ? Or, on the other hand, does de- 

 velopment involve the operation of specific 

 vital entelechies or powers that are without 

 analogue in the automaton and are not in- 

 herent in any primary material configura- 

 tion of the egg? This question, I hardly 

 need say, is included in the larger one, 

 whether the vital processes as a whole are 

 or are not capable of mechanical explana- 

 tion. As a problem of embryology it is 

 very closely connected with that of prefor- 

 mation or epigenesis, and in point of fact 

 the two have always been closely associated. 

 Evidently, by its very form of statement, 

 any theory of preformation or prelocaliza- 

 tion in the germ assumes at least a mechan- 

 ical basis for development, ?. c, a primary 

 material configuration upon which the 

 form of development in some measure de- 

 pends. AVith theories of epigenesis the 

 ease is not so clear; for such theories may 

 or may not be mechanical. Without fur- 

 ther preamble I now ask your attention to 

 certain facts which will place clearly before 

 us the form in which these time-honored 

 problems appear to us to-day. 



It is a familiar fact that development 

 begins with the progressive segmentation 

 or division of the egg into cells, which, con- 

 tinually increasing in mmiber, finally build 

 up the body of the embryo. Until com- 



