574 ON INCREASE IN COMPLEXITY [pt. iii 



the work of Streeter on the amphibian ear, which showed that the 

 membranous labyrinth is self-differentiating, and of Luther, which 

 showed that, on the contrary, the cartilaginous auditory capsule is 

 dependent-differentiating, i.e. in relation to the auditory vesicle as 

 to a second-grade organiser. Again, in Bombinator the visceral rudi- 

 ments are self-differentiatory, according to Holtfreter. 



Tissue culture methods have been already referred to as important 

 in the analysis of the differentiatory power of a given anlage. These 

 may be of various kinds. One of the most valuable is the transplanta- 

 tion of the tissues of bird embryos on to the chorio-allantoic membrane 

 of another c^gg, where it can easily be seen whether or not they 

 continue to develop in isolation, and, if they do, what they develop 

 into. In this way Danchakov found that the blastoderm of the chick 

 develops properly in isolation, but only after some hours. The degree 

 of differentiation here depends entirely on the age of the embryo 

 from which the piece is taken. Thus Hoadley found that pieces of 

 an embryo only 4 hours old will, when transplanted on to the chorio- 

 allantoic membrane, produce an eye with pigment cells only; if the 

 age of the embryo is 6 hours, an eye with pigment and retinal cells 

 will be produced, and, if the age of the embryo is 8 hours, an eye 

 with pigment and stratified retina will result. At 10 hours the 

 primitive groove is formed, and, if a piece be taken from an embryo 

 after the beginning of somite formation, a completely self-differen- 

 tiating eye will develop on the chorio-allantoic membrane. Other 

 workers have made similar experiments with different regions of the 

 chick embryo as follows: limbs and limb girdles — Murray & Huxley; 

 Selby & Murray; and Spurling; eye — Barfurth & Dragendorff; 

 head — Lillie, and Danchakov; spinal cord — Agassiz & Danchakov; 

 nose, ear and mesencephalon — Hoadley; metanephros — Atterbury; otocyst — 

 Fell; gonads — Minoura; mesonephros and primordial germ cells — 

 Humphrey; various tissues — Hiraiwa; heart — Danchakov & Gagarin. 

 The other main form of explantation work is the in vitro technique 

 of tissue culture. This has been used to study self-differentiation 

 by Reinhov, who found that the kidney of an embryonic 

 chick would develop glomeruli, tubules, and capillaries in vitro, 

 and by Strangeways, who got good differentiation in embryonic 

 cartilage. Working with the same technique Strangeways & 

 Fell showed that the limb buds of embryonic chicks have only 

 a small power of self-differentiation in vitro (or in vivo in trans- 



