198 THE MOSAIC STAGE OF DIFFERENTIATION 



A similar total and permanent absence of a whole organ has been 

 obtained by extirpation of the presumptive limb-area, both in 

 Urodela^ and in the chick. ^ In these cases, the presumptive limb- 

 area is a discoid region of mesoderm and ectoderm, with no visible 

 differentiations. As will be seen later (p. 420), in adult Urodela, 

 regeneration of a limb will occur even when the whole limb and its 

 skeleton, including the girdle, is extirpated, provided that the 

 sympathetic nervous system is left intact.^ No experiments seem to 

 have been carried out to discover whether any regeneration would 

 occur in an animal lacking a limb owing to early embryonic 

 extirpation of the limb-area, if the region on the flank where the 

 limb ought to be were removed at the adult stage ; we may presume, 

 however, that there would be no regeneration. (See fig. 22, p. 56.) 



The hypophysis arises from a rudiment of ectoderm on the front 

 of the head. This rudiment can be extirpated from Anuran larvae 

 at the tail-bud stage, and it is found that the larvae^ which ulti- 

 mately develop are normal except that they lack the pituitary gland. ^ 



Even the blood in the Anuran embryo has a definite and localised 

 rudiment, situated in the mesoderm of the splanchnopleur, in the 

 mid-ventral line, anterior to the heart. If this rudiment is extir- 

 pated completely from embryos of Rana temporaria at the early 

 tail-bud stage, no erythrocytes are formed, and in cases of partial 

 extirpation the quantity of erythrocytes produced is proportional 

 to the amount of the rudiment which is left.^ 



The Ascidians provide another case of animals which in the adult 

 state are capable of extensive and far-reaching regeneration and 

 reorganisation, but which in the early stages of embryonic de- 

 velopment are unable to make good any loss which the various 

 determined regions may sustain.'^ 



^ Harrison, 1915. ^ Spurling, 1923. 



^ Bischler, 1926. 



^ Incidentally, it may be mentioned that such larvae are of great interest also 

 from another point of view, for they are incapable of producing the pituitary 

 hormones, and are therefore permanently light in colour, and incapable of normal 

 metamorphosis . 



^ Smith, 1920. ^ Frederici, 1926. 



' Conklin, 1905, 1906; Huxley, 1926. 



