A PRELIMINARY EXPERIMENTAL ANALYSIS 53 



The vital importance of the organiser for development is shown 

 by the classical experiment of separating the first two blastomeres 

 of the newt's egg. If the plane of the first cleavage separates the 

 future right and left halves of the body, both blastomeres will re- 

 ceive a portion of the organiser region, and both will organise them- 

 selves and produce miniature but otherwise normal embryos.^ 

 But if the first cleavage separates future dorsal and ventral halves, 

 only the dorsal half will produce an embryo ; the ventral half under- 

 goes cleavage and makes an abortive attempt to produce germ- 

 layers, but develops no further^ (fig. 20). The same is true in the 

 case of the frog.^ 



The action of the organiser raises a number of important 

 problems which will receive more detailed consideration in a 

 subsequent chapter. For the moment, attention may be focussed 

 on the light which these phenomena throw on the analytical study 

 of development. 



§7 



It has been seen that the newt's egg when fertilised has already had 

 two determinations imposed upon it : that of polarity and that of 

 bilateral symmetry. As a result of these determinations, one region, 

 the future organiser, is localised and apparently fully determined 

 at very early stages. Until a certain time, which is roughly half-way 

 through the process of gastrulation, the various other regions of the 

 embryo are still plastic, although they are presumably passing 

 through the preliminary stages of chemo-differentiation. But the 

 time comes when they, too, are irreversibly determined to follow 

 the course of differentiation which characterises each part in normal 

 development. 



The terms mdepende?it or self-differentiation and dependent differ- 

 entiation were introduced by Roux to characterise these two types 

 or phases of diff"erentiation. In Amphibia before gastrulation, all 

 regions save that of the organiser show dependent differentiation : 

 their developmental fate is dependent upon and conditioned by 

 factors external to themselves — in this case the presence of an 

 organiser in a particular spatial relation with them. This is proved 



^ Herlitzka, 1896; Spemann, 1903. 2 Schmidt, 1930, 1933. 



