406 THE HEREDITARY FACTORS AND DIFFERENTIATION 



may concern not only such characteristics as pigmentation, but also 

 cell-size, specific growth-intensity, specific structures (see Chap, vi, 

 p. 142 and Chap, vii, p. 236), or the 

 time-relations of development. 



An example of this last type is 

 provided by experiments in which a 

 portion of presumptive neural tube 

 material of Triton taefiiatus is grafted 

 into the side of an embryo of Triton 

 cristatus. It may there undergo diflPeren- 

 tiation into gills, but such gills pre- 

 serve a feature of their specific origin, 

 although the tissues from which they 

 have arisen would normally never have 

 given rise to gills. In Triton taeniatus 

 the gills develop relatively earlier than 

 in cristatus^ and in the experiment just 

 described the gills which are formed 



from the graft oUaeniatus tissue show Sectorial limb-chrmaera 

 a greater precocity of diflFerentiation axolotl, produced by combining 

 than the host cristatus gills of the other ^^^ ^«^^^! ^^^^ 5^/ hind-limb 



. ^ . . . regeneration- bud irom a black 



Slde.l The taeniatus tissue, m its specimen with the ventral half of 



diflFerentiation into a structure which a hind-limb regeneration-bud 



1 , ,1 1 c 1 left in situ on a white speci- 



It would normally never have formed, ^^^. one year after operation. 



is still controlled by certain of its (Redrawn after Schaxel, Arch. 



hereditary factors. Still more demon- ^«^^^^^^^- l, 1922.) 

 strative results have been obtained by xenoplastic grafting between 

 Anura and Urodela (Chap, vi, p. 142). Here, then, is additional 

 evidence of the fact that the hereditary equipment of all the cells 

 of the organism is the same (see Chap, v, p. 85). 



It is possible to make up a compound embryo by grafting to- 

 gether an anterior half- embryo of Rana virescens and a posterior 

 half-embryo of Rana palustris, or vice versa. The compound 

 organism behaves as a unit in regard to its general physiology and 

 can undergo metamorphosis and develop into a full-grown frog. 

 But the two components retain some of their specific characters, 

 not only as regards pigmentation, but also as regards structural 

 ^ Spemann, 1921. 



Fig. 195 



