CHAPTER VIII 

 PFLUGEPCS EXPERIMENTS ON THE FROG'S EGG 



Ix order to discover how far the development depends on 

 the surrounding conditions to which the egg is subjected, we 

 must change those conditions and observe the result. In this 

 way we may hope to find out to what extent the phenomena of 

 development are dependent on conditions outside of the egg, 

 and how far they result from the egg itself. 



PfiiAger made, in 1883, a brilliant series of experiments that 

 have been the point of departure for much of the later work 

 on the frog's egg; therefore, in this chapter, I shall give a 

 somewhat detailed account of Pfliiger's work. The results 

 are arranged in an order different from that followed by 

 Pfliiger, with the hope of making clearer a necessarily brief 

 abstract. 



The following orientation of the egg will facilitate the de- 

 scription of the experiments. If the middle 1 point of the black 

 hemisphere of the frog's egg (the " black pole "') is imagined 

 to be connected with the analogous point of the white hemi- 

 sphere (i.e. with the %4 white pole") by a straight line passing 

 through the centre of the egg, this line forms the primary diam- 

 eter or primary axis of the egg. An imaginary primary equator 

 and a system of parallels and meridians belong to such a diam- 

 eter. When the frog's egg segments, the first two cleavage- 

 planes are found to be vertical in whatever position the egg 

 may lie. The line of intersection of these first two planes 

 passes through the centre of the egg, forming what we may 



1 Pfliiger does not notice that in the normal egg at rest this " middle part " is 

 not necessarily the highest part of the egg. Correspondingly, the lower pole need 

 not be the lowest point of the egg. For the present, however, we must disregard 

 this distinction. 



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