DEVELOPMENT 275 



which, though long since swept aside in their original form as a 

 result of the increase of knowledge, raised a problem that is still 

 before the embryologist to-day. 



In brief, one view virtually denied development by maintaining 

 that the adult organism is nearly or completely formed within the 

 germ, either in the egg or the sperm, which merely by expansion, 

 unfolding, and growth gives rise to the new generation. In this 

 first crude form the preformation theory demanded the 'encase- 

 ment ' of all future generations one within another in the germ of 

 existing organisms, so that when it was computed that the pro- 

 genitor of the human race must have contained some two hundred 

 million homunculi (a conservative estimate, to say the least) the 

 reductio ad absurdum was irresistible. 



The other view was reached by careful studies on the transforma- 

 tion of the Hen's egg into the chick which soon made it clear that 

 the chick is not preformed in the egg. The embryo arises by a 

 gradual process of progressive differentiation from an apparently 

 simple fundament — it is a true process of development, or epi- 

 genesis. But the upholders of epigenesis versus preformation were 

 before long beyond their depth and in danger of attempting to 

 get something out of nothing — lost in the miraculous. 



A statement in such succinct form tends to accentuate the cru- 

 dities of these two conflicting views — "preformation explaining 

 development by denying it and epigenesis explaining development 

 by reaffirming it" — and it may be well to remark that the early 

 embryologists with the means at their command faced a stupendous 

 task of which only recent work has brought a full appreciation. 



The path to progress cleared by the realization that adult 

 structures are not preformed as such in the egg, and that develop- 

 ment is not an expansion but the formation — the ' becoming ' — 

 by an orderly sequence of events of structures of great complexity 

 out of apparent simplicity, the problem of the embryologist was to 

 determine what the egg structure actually is, and how it is related 

 to that of the adult. To trace the development of these studies 

 would involve the history of embryology since the formulation of 

 the cell theory. We must confine ourselves to the bare statement 

 of the new guise in which the old theories of preformation and 

 epigenesis confront us to-day as a result of recent research. 



The reader already recognizes the fertilized egg as a cell, with 

 its nucleus comprising a complex of quite definite elements — the 



