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harmonious relation to one another. The same kind of thing is true in the 

 opposite combination of a small punctatum eye-cup with a large tigrinum 

 lens; the growth-rate of the eye-cup is increased and that of the lens 

 decreased until the two fit. Again the mechanism of the effect is quite 

 obscure. Weiss (i 949^2) has suggested that it may be a matter of tension 

 exerted on the growing edge of the retina. In a combination where the 

 lens is too small, it will permit some of the vitreous humour to flow out 

 of the eye-cup and thus the tension will be reduced and the growth rate 

 lowered. On the other hand, an over-large lens will exert a radial pressure 

 against the edges of the retina and this might lead to an increased growth 

 rate (Figs. 13.9, 13.10). 



Figure 13.9 

 Reciprocal effects of lens and eye-cup. Figures a and h show the eyes of a 

 fairly young tadpole of Tritums tacniatiis in which the lens of the left eye {a) 

 has been induced out of axolotl ectoderm grafted into the region; the lens 

 is much too large in relation to" the eye-cup. Figures c and d show a 

 later stage from a similar experiment (at a lower magnification) ; the lens 

 derived from axolotl tissue (in c) is still larger than the corresponding 

 taeniatus lens, but it has caused its associated eye-cup to grow faster than 

 normal, so that the relative sizes are nearly adjusted to one another. ("After 

 Rotmann 1939) 



