10 HEREDITARY CHARACTERS 



by the other cells in the body. This theory appears to 

 claim, in fact, that each collection of idioplasm present in a 

 fertilised ovum contains a sufficient number of different 

 substances to provide for all the different tissues building 

 up the body. The only difference in the end between this 

 and the germinal localisation theory appears to be that 

 instead of particular areas we have particular substances. 



In spite, however, of the inherent improbability of every 

 character being usually represented by. a particular entity, 

 whether of area or substance, there can be but little doubt 

 that under normal conditions a particular area of a fertilised 

 ovum does generally produce a particular part of the body 

 in certain animals. An experiment performed by Roux on 

 the frog's egg is a very good illustration of this. 1 He took 

 the fertilised ova of the frog which had reached the two-cell 

 stage, that is, the original cell of the fertilised .ovum had 

 divided into two cells. In each case he killed one of these 

 cells with a hot needle, and in some cases the remaining 

 cell developed into a half-embryo possessing only the right 

 or left side, according to which of the two sides had been 

 destroyed. On the other hand, Roux also found that in his 

 experiments, when carried on further, the existing half- 

 embryo restored more or less completely the missing half. 

 Later experiments by other observers were made with the 

 eggs of several other animals, which appear to show that, in 

 the earlier stages of development at any rate, all the cells 

 into which the fertilised ovum divides retain the power of 

 producing all the tissues that would under ordinary circum- 

 stances be produced by the fertilised ovum itself. Driesch, 

 Morgan, Wilson, Zoja, and others have separated the cells 

 produced by the division of the fertilised ovum, when de- 

 velopment had gone as far, in some cases, as the sixteen-cell 

 stage. 2 The cells thus shaken apart have developed into 



1 Roux, W., "tiber das kiinstliche Hervorbringen halber Embryonen durch 

 Zerstoning einer der beiden ersten Furchuugskugeln," &c., Virdunc's Archives, 

 114, 1888. 



2 Driesch, H., Analytische Theorie der Organischen Enticickluny, Leipzig, 

 1894; Morgan, T. H., "Experimental Studies on Teleost Eggs," Anat. Anz., 



