THE MOSAIC STAGE OF DIFFERENTIATION 249 



Amblystoma above-mentioned, the eye-region when not underlain 

 by gut-roof is found to be still capable of regulation within itself 

 at the neurula stage. The results of the experiments on Pelobates 

 are apparently to be explained only on the supposition that chemo- 

 differentiation of the eye-region in this form occurs more slowly. 



When these eye-cups are very small, it is frequently found that 

 they are abnormally proportioned in that they may have too many 

 or too few tapetum cells ; too much or too little optic stalk or retina, 

 compared with the proportions in which these constituents are 

 found in the normal eye-cup. The explanation appears to be that 

 the various constituents of the eye are now separately and in- 

 dividually determined. The cut, going haphazard through the eye- 

 rudiment, will often separate parts which possess the prospective 

 constituents of a normal eye in abnormal proportions. Similar 

 results are obtained from experiments on Rana palustris in which 

 incomplete eye-rudiments are grafted into Various positions. Here, 

 even the various retinal layers appear to be determined^ (fig. 121). 



§ 10 



Yet another conclusion of importance emerges from the experiment 

 mentioned above (p. 245). It w^ill have been noticed that although 

 some of the miniature eyes are abnormal in the proportions of their 

 constituent parts, they nevertheless round themselves off into little 

 spheres resembling normal optic vesicles, and some at any rate of 

 these little vesicles become invaginated to form cups. In some 

 cases, cups formed entirely of tapetum without any retinal tissue 

 are produced. In other words, the processes of morphological 

 diiTerentiation, or production of form, are not dependent on the 

 histological differentiation of the tissues which they are moulding. 

 A similar conclusion can be drawn from the results of other ex- 

 periments, in which it has been shown that portions of the neural 

 fold region, or of the heart, or gut region, roll themselves up into 

 tubes in spite of the fact that their histological differentiation may 

 be abnormal.^ 



At the same time, other work, and on the most diverse groups, 

 has shown that the histological differentiation of the tissues may 



^ Lewis, IQ08. 



" Roux, i885;Ekman, 1924; Stohr, 1925; Boerema, 1929; Holtfreter, 193 ib 



