IV. a INTERACTIONS OF THE PARTS 275 



Pluteus is either distorted, in the first two cases, or absent, as in 

 the last. The distortion takes the form of a multiplication of 

 the triradiate spicules, and the arms are correspondingly multi- 

 plied. In water devoid of CO 2 there are no spicules and no arms. 1 

 Herbst has therefore urged that normally the outgrowth of the 

 ciliated ring into the arms is due to a stimulus thigmotropic, 

 perhaps exerted by the tip of the spicule. The converse of this 

 is seen later on when the arms diminish in length as the calcium 

 carbonate of the Pluteus skeleton is made use of by the developing 

 urchin. 



The development of the lens of the vertebrate eye has also 

 been asserted to be due to a contact stimulus exerted by the 

 optic cup upon the overlying ectoderm. Spemann was the first 

 to show that when the medullary plate of the Frog (Ranafusca) 

 was injured in front of the optic vesicle, the optic cup (if 

 developed at all) may or may not come in contact with the 

 ectoderm, the latter being, at this point, uninjured. In the first 

 case a lens is formed, in the second not. Similar experiments 

 have been carried out by Lewis on other species of Frog and on 

 Amblystoma, with similar results. Lewis has also shown that 

 a lens will be formed from any patch of ectoderm taken from 

 some other part of the body and grafted over the optic cup. 

 Schaper's evidence in support of this conclusion is doubtful. 

 This author removed the whole of the nervous system from a 4 mm. 

 tadpole by a horizontal cut with the exception alone of the 

 downwardly growing fore-brain and optic vesicles. The wound 

 healed and development continued. The optic cup was solid, 

 being filled with a mass of degenerating cells. A lens was 

 formed opposite the upper edge of the optic cup. It did not, 

 however, separate from the ectoderm, but differentiated in situ, 

 with development of lens fibres. Hence Schaper argues that the 

 formation of the lens is a process of self-differentiation. The 

 contact of the optic cup is, however, as he admits, in this case 

 not excluded. 



A more serious objection is raised by Miss King, who has found 

 (in the Frog) that even when, owing to a prior injury, the optic 

 cup is far away from the skin, or in some cases absent altogether, 



1 See above, Ch. III. 8, Figs. 73, 74. 

 T 2, 



