2o8 THE MOSAIC STAGE OF DIFFERENTIATION 



been grafted, the wing-rudiment differentiates according to the sex 

 of its donor. ^ Recent work on various Insects indicates that after 

 a certain stage the embryo is a mosaic of chemo-differentiated re- 

 gions, although the details of the determination-process differ 

 considerably from those found in Amphibia.^ 



In Cephalopods it has been shown that fragments of embryos 

 cultivated by explantation methods continue their differentiation 

 as if they formed part of the whole organism.^ These experiments 

 were undertaken after visible differentiation had appeared ; others, 

 however, indicate that the embryo passes into the mosaic chemo- 

 differentiated stage just before visible differentiation occurs.* This 

 would, in general, be similar to the state of affairs in Amphibia. 



Another remarkable case of self-differentiation during the mosaic 

 stage of development concerns the self-orientating properties of 

 the auditory vesicle in Amphibia. If at the stage when it is a simple 

 vesicle, the auditory sac is turned upside down and left in situ, it 

 often rights itself by rotation, so that its dorso-ventral axis con- 

 forms to that of the whole animal.^ The suggestion that the ear- 

 vesicle rights itself because it only fits properly into the neigh- 

 bouring structures when it is in its normal position must be dis- 

 carded, because a right ear-vesicle, grafted upside down in the space 

 vacated by an extirpated left vesicle, rotates and becomes right way 

 up and right way out, but as the vesicle retains its laterality, it 

 develops with its normally anterior side pointing backwards in the 

 animal. It thus rights itself in respect of its dorso-ventral axis in 

 spite of the evident misfit which results. Further, an inverted 

 vesicle of Rana will right itself in Amblystoma, and vice versa.^ 



The rotation of the ear-vesicle may be impeded by special local 

 conditions of the experiment, but when it occurs it takes place 

 gradually, and, to all appearances, in relation to gravitational 

 stimuli. The ear is, of course, an organ whose function it is to detect 

 the direction of maximum gravitational attraction, and, should the 

 supposition be verified that the righting effect is directed by gravi- 

 tation, the ear- vesicle in Amphibia may be regarded as determining 

 its orientation independently of the rest of the organism. Un- 



^ Kopec, 1911, 1913. " Seidel, 1929, 1931; Reith, 1932; Pauli, 1927- 



3 Ranzi, 193 1. * Ranzi, 1928. 



^ Streeter, 1906, 1914; Spemann, 1910. *' Ogawa, 1921. 



