THE MOSAIC THEORY OF DEVELOPMENT. 3 



ence of gravity and mechanical pressure upon the segmenting 

 ova of the frog. These pioneer studies formed the starting- 

 point for a series of remarkable researches by Roux, Driesch, 

 Born and others, that have absorbed a large share of interest 

 on the part of morphologists and physiologists alike ; and it 

 is perhaps not too much to say that at the present day the 

 questions raised by these experimental researches on cleavage 

 stand foremost in the arena of biological discussion, and have 

 for the time being thrown into the background many problems 

 which were but yesterday generally regarded as the burning 

 questions of the time. It is the purpose of this lecture to 

 consider, briefly, the most central and fundamental subject of 

 the current controversy. 



It is an interesting illustration of how even scientific history 

 repeats itself that the leading issue of to-day has many points 

 of similarity to that raised two hundred years ago between the 

 prae-formationists and the epigenesists. Many leading biolo- 

 gical thinkers now find themselves compelled to accept a view 

 that has somewhat in common with the theory of prse-forma- 

 tion, though differing radically from its early form as held by 

 Bonnet and other evolutionists of the eighteenth century. No 

 one would now maintain the archaic view that the embryo 

 prae-exists as sucJi in the ovum. Every one of its hereditary 

 characters is, however, believed to be represented by definite 

 structural units in the idioplasm of the germ-cell, which is 

 therefore conceived as a kind of microcosm, not similar to, 

 but a perfect symbol of, the macrocosm to which it gives rise 

 (Hertwig). In its modern form this doctrine was first clearly 

 set forth by Darwin in the theory of Pangenesis ('68). Twenty 

 years later ('89) it was remodeled and given new life by Hugo 

 de Vries, in a profoundly interesting treatise entitled Intra- 

 cellular Pangenesis and in its new form was accepted by 

 Oscar Hertwig, and pushed to its uttermost logical limit by 

 Weismann. Kindred theories have been maintained by many 

 other leading naturalists. 



The considerations which have led to the rehabilitation of 

 the theory of pangenesis are based upon the facts of what 



