196 



rhomb 



iF.F. 



opt. st. opt. c 



Fig. 93 

 Self-differentiation in fragments of the vertebrate embryo. The head-region of 

 a 24-hour chick embryo was grafted on to the chorio-allantois of another egg and 

 allowed to develop for 4 days. Above, eye-region enlarged. Below, longitudinal 

 section. It differentiated the main regions of the brain {rhomb . rhombencephalon ; 

 Twj;^/. myelencephalon ; m.6.mid-brain;/.6. fore-brain; e^. epiphysis ; inf.f.inixxn- 

 dibulum), together with an eye showing optic stalk (opt.st.), optic cup (opt.c), and 

 lens, /; bl.v. blood-vessel. The histogenesis was normal, but the form-differen- 

 tiation, notably of the eye, and also of the brain (note b. bar across cavity of fore- 

 brain), abnormal in many respects. (From Murray and Huxley, Brit.Journ. Exp. 

 Biol. Ill, 1925.) 



