326 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



that multiple development can be obtained by cutting the un- 

 fertilised egg and inseminating the fragments (p. 120); by isolating 

 1/2 or 1/4, and in some cases even 1/8 blastomeres (p. 97); or by 

 cutting and breaking the blastula into fragments (pp. 81, 89). 

 The most significant example of the multiplication of potencies in 

 the early egg is perhaps the production of double monsters from 

 inverted frog's eggs (p. 94). In this case there is no spatial isola- 



Fig. 151 

 Multiple potentiality in head-field in the Planarian Dendrocoelum lacteiun. The 

 anterior end was partially slit by a number of cuts ; the organism has produced 

 ten heads. (Redrawn from Korschelt, Regeneration und Transplantation, 1927, 

 fig. 269, p. 444; after Lus.) 



tion of fragments; a physiological isolation between two active 

 regions is brought about by the intercalation of a mass of inert yolk. 

 The coalescence of two eggs to produce a single unitary embryo is 

 a converse result of the same principles. Further, just as two- 

 headed Planarians or bifurcated regenerated limbs can be produced 

 by operations, so can two-headed newt embryos be produced by 

 partial constriction in the 2-cell stage (pp. 75, 350). 



