206 THE MOSAIC STAGE OF DIFFERENTIATION 



grafted on to the chorio-allantoic membrane show the same specific 

 differences between the growth- capacities of the right and left 

 sides. ^ In the mammal, also, the tissue culture of embryonic 

 material provides evidence of self-differentiation. ^ Portions of 

 rabbit embryos 9 to 12 days old, cultured in vitro, reveal the 

 mosaic character of development : the various rudiments differen- 

 tiate independently,^ just as similar fragments do when grafted. 

 The growth-partition coefficients of Urodele limbs are inherently 

 determined.* 



A striking case of independent differentiation is provided by the 

 silkworm. The wing rudiments in Lepidoptera are protruded from 

 the surface during pupation, and the pupal case has pockets 

 into which the wings fit snugly. In the silkworm, a mutation has 

 been found which results in the animal being wingless. Neverthe- 

 less, the pupal cases of such mutants possess the characteristic 

 pockets, although no wings project into them.^ Similar occur- 

 rences have been observed in Papilio dardanus where in the female 

 the wings may have no tails, but pockets for them are provided 

 in the pupal wing-cases.^ 



This case is of considerable theoretical interest, for, in general, 

 when two structures are closely associated topographically, it is 

 found that the differentiation of the one is often dependent on the 

 other. Numerous examples have been given in Chap, vi : we may 

 recall the eye-cup and the lens of Rana fiisca ; the eye and the con- 

 junctiva ; the tympanic cartilage and the tympanic membrane ; the 

 skeleton and the arms of the pluteus ; the hydrocoel and the am- 

 niotic cavity of the echinoid rudiment. In Rana esculenta, however, 

 as we have seen (p. 186), the eye-cup and the lens are independent 

 from a very early stage, and in this they resemble the wing and 

 wing-case of the silkworm. 



Further evidence of the self- differentiating capacity of the wing- 

 rudiment in Lepidoptera is provided by the experiments of grafting 

 the wing-rudiments of caterpillars from one sex into the other. The 

 fully developed wing is markedly different in the two sexes, and 

 it is found that regardless of the sex of the host into which it has 



^ Willier, 1927. ^ Waddington and Waterman, 1933. 



^ Maximow, 1925. ^ See Huxley, 1932, Chap. vi. 



^ Goldschmidt, 1927, p. 203. ^ Lamborn, 1914. 



