3<"0 The American Naturalist [Maj, 



just ventral to the equator of the egg. Normally the blasto- 

 pore appears just ventral to the equator and passes, so Pfluger 

 maintained, across the white pole to the opposite side of the 

 equatorial region. In unusual positions the blastopore also 

 appears just beneath the horizontal equator, whatever part of 

 the egg this equator may be in the given inclined position. 



Pfluger came thus to regard the essential elements of an egg 

 as having no more prearranged relationship in position to the 

 part of the future embryo than do "snowflakes to the ava- 

 lanche they may give rise to." Gravity acting to arrange the 

 parts of an egg according to circumstances, much as snowflakes 

 may be collected by gravity to form an avalanche, under 

 certain circumstances. 



The fundamental character of these experiments seems to 

 be upheld by the fact presented in a third paper 4 that eggs 

 thus held in oblique positions actually developed as far as the 

 adult shape, quite normal toads being reared from eggs of 

 ]!'iml,;,i<itcr igneus. 



Moreover other forces than gravitation act in the same 

 directive fashion upon the cleavage, for Pfluger mentions his 

 repetition of certain experiments of Rauber upon trout eggs, 

 using eggs of both Rana and Bombinator and finding that in a 

 rotating machine the centrifugal force controlled the appear- 

 ance of cleavage planes. 



Frog's eggs compressed between vertical plates of glass 

 cleave at right angles to the plates and generally in the 

 vertical plane also, but in exceptional cases al all angles to 

 the vertical plane: here pressure is assumed as a sufficient 

 determining influence. 



The frog's egg is thus regarded as directly influenced by 

 external forces and fundamentally altered in the arrange- 

 ments of its constituents. The egg is like a mass of porridge 

 with its outer part firmer and its main mass perhaps traversed 

 by some reticulum of firmer material, but still so mobile that 

 the heavier yolk can sink to the bottom and the less heavy 

 remain above it, much as sediment falling in a liquid. 



u„t!T fl ?3 er p-Y eber , die EUiwirkung der Schwerkraft and andcrer Beding- 



ungen auf die Richtung der X. ; , ; s . . , ,,. niu-tilb. 



