34 THE PHYSICAL BASIS OF LIFE 



It was a strange notion of the seventeenth century that 

 the embryo exists from the beginning, preformed in the 

 egg, and has only to nnfold itself during development; 

 and out of this phantasy grew one of the most singular 

 controversies in the history of modern science. It was 

 halted in the middle of the eighteenth century by the ob- 

 servations of Cas]3ar Friederich Wolff, who decisively 

 overthrew the doctrine of preformation in its original 

 form. Modern research, nevertheless, has dressed this no- 

 tion out with a new disguise in the conception of prelo- 

 calization in the egg. The embryo, it is said, is already 

 present in the egg, not indeed in its completed form but 

 as it were blocked out "in the rough" in the cytoplasm, so 

 that development has only to impress upon it the finishing 

 touches. This conception grew out of certain well deter- 

 mined facts, to the estal^lishment of which experimental 

 embryology, cytology and genetics all contributed. 

 Prominent among them is the fact, demonstrated by stud- 

 ies on certain hybrids, that certain general features in the 

 early develoj)ment seem to be determined by the cyto- 

 plasm (ooplasm) of the egg alone, uninfluenced for a time 

 by the sperm. 



In harmony with this, up to a certain point, are the 

 following facts. Both observation and experiment have 

 conclusively proved that during the cleavage of the egg 

 its substance undergoes a definite differential distribu- 

 tion to the embryonic cells, being parceled out, as it were, 

 in such manner that each cell receives a particular allot- 

 ment of specific materials by which its immediate de- 

 velopment is determined or conditioned. The pattern of 

 cleavage, thus represents a kind of mosaic-work of such 

 materials localized within the boundaries of its compo- 

 nent cells. It was at first assumed by Roux and by Weis- 

 mann that the determining materials thus segregated 



