252 THE MOSAIC STAGE OF DIFFERENTIATION 



entiation characteristic of their normal fate, but the morphological 

 differentiation is not always achieved. If, on the other hand, such 

 presumptive pieces are taken from an early gastrula (i.e. before the 

 organiser has been invaginated) and grafted, they will undergo the 

 morphological differentiation of their new surroundings: pre- 

 sumptive neural fold tissue in an epidermal area will remain flat, 

 while presumptive epidermis in the neural fold area will become 

 folded up into a neural tube. But, in spite of the morphological 

 differentiation which these pieces are forced to undergo, they retain 

 some of the histological characteristics of their normal prospective 

 fates. ^ 



In these experiments we have on the one hand the fact that histo- 

 logical differentiation can take place without morphological, and 

 on the other, the fact that morphological differentiation when forced 

 upon a piece of tissue does not entirely obliterate its presumptive 

 histological characteristics. It is necessary to conclude, therefore, 

 that in these cases, morphological and histological differentiation 

 are independent of one another. 



There are other facts which point to the same conclusion. In the 

 larva of the sea-urchin, for instance, some histological differentia- 

 tions (apical organ) take place without any antecedent form- 

 changes of the tissue in question. In amphibian material, the 

 results of experiments involving the culture of pieces of tissue in 

 vitro likewise point to the independence of histological and morpho- 

 logical differentiation. We need only point to the instances men- 

 tioned above in Chapter iii (p. 50), in which pieces of tissue taken 

 from the blastula or early gastrula show far-reaching powers of 

 histological differentiation without having undergone any specific 

 form-changes, or any morphological differentiation. 



Perhaps the most striking demonstration of the independence of 

 morphological and histological differentiation is provided by those 

 cases in which an insect tgg (Platycnemis) gives rise to two embryos 

 as a result of a transverse discontinuity in the blastoderm. The two 

 embryos develop, each from its ventral surface in the normal way, 

 and they are situated back to back. One is larger than the other, 

 and when it folds up its sides to form its dorsal surface, it actually 

 encloses its smaller brother within itself, and compels it to perform 

 ^ Lehmann, 1928 b, 1929. 



