FIELDS AND GRADIENTS IN NORMAL ONTOGENY 329 



meres of the 2-cell stage. ^ Here also may be included those cases 

 in which for reasons at present unknown the heavily yolked egg of 

 fish and of birds may exceptionally possess two blastoderms, and 

 perhaps cases of double monsters in scorpions.^ 



Twinning in the restricted sense, however, is the result of a 

 dichotomy setting in, not during the earliest stages of cleavage, 

 but during later stages of development, and resulting in definite 



Kflgf.i 

 Kfbgf.a 



Fig. 152 



Incipient twinning mechanically produced. Ventral view of Triton embryo 

 from an egg slightly constricted in pre-gastrulation stage, showing slight anterior 

 doubling. Kfbgf.a, outer gill-filaments ; Kfbgf.i, inner gill-filaments ; oc, inner 

 eyes ; olf, olfactory pits ; * pigment overlying heart-rudiments. (From Spemann, 

 Arch. Entzvmech. xvi, 1903.) 



fission of one embryo into two or more. Here belong the cases of 

 twinning as found regularly in the armadillo (here resulting in the 

 formation of four or eight embryos),^ occasionally in other mam- 

 mals including man (leading to the production of so-called identical 

 twins), or in birds or earthworms leading to the production of 

 double monsters: experimentally leading to the production of 

 double monsters in the frog after reversal of the tgg, in Fiindulus 

 and trout after subjecting the tgg to cold or oxygen-deficiency 

 ^ Newman, 1923. - Brauer, 1917. ^ Newman, 19 17, 1923. 



