Organisms from Eggs 141 



power of "regulation" of the blastomeres should differ 

 according to the degree of fluidity and the possibility 

 of different layers separating, and this assumption 

 is apparently supported by facts. The first plane of 

 segmentation of the egg is usually the plane of sym- 

 metry of the later organism and where the degree of 

 fluidity is less than in the sea-urchin egg, a separation 

 of the two first blastomeres should easily result in the 

 formation of two half -embryos instead of two whole 

 embryos. 



This is the case for the frog's egg as Roux showed in 

 a classical experiment. Roux destroyed one of the 

 two first cleavage cells of a frog's egg with a hot needle 

 and found that as a rule the surviving cell developed 

 into only a half -embryo. * The frog's egg consists of 

 two substances, a lighter one which is on top and a 

 heavier one below. Although viscous, the two sub- 

 stances are not too viscous to prevent a flow if the egg 

 is turned upside down. O. Schultze found that if a 

 normal egg is turned upside down in the two-cell stage 

 and held in that position, two full embryos arise, one 

 from each of the two blastomeres. Through the flow 

 of the lighter liquid in the egg upwards the two halves 

 of the protoplasm on top become separated and de- 

 velop independently into two whole embryos instead 

 of into two half-embryos. In Roux's experiment this 

 flow of protoplasm was avoided. Morgan showed that 



1 Roux, W. f Virchaw's Archiv, 1888, cxiv., 113. 



