THE MOSAIC STAGE OF DIFFERENTIATION 



233 



given rise to epidermis.^ This shows that more cells are capable 

 of ear-formation than normally exert this capacity. 



This is confirmed for Urodeles by experiments on Amhlystoma. 

 At later stages, when the rudiment has invaginated to form the ear- 

 vesicle, the power of ear-formation is lost by the neighbouring 

 epidermis,^ for if the vesicle is extirpated it is not regenerated. 



Just as in the case of the limb-field, the ear-field very soon shows 

 a polarisation. If a piece of the ear-area of Rana nigromaculata (at 

 the stage when the rudiment is just thickened) is rotated through 

 180°, the auditory vesicle which sub- 

 sequently develops is reversed. Further, 

 a piece of the ear-area of one side grafted 

 on to the opposite side of the body de- 

 velops with the asymmetry of its side 



of origin.^ This shows that the rudi- WHP- * - -^ — ^^ 



ment was already determined as regards 

 two at least of its axes. 



The gill-region in Amphibia also 

 constitutes a field in the ectoderm of 

 the embryo. At the early neurula stage 

 in embryos of Amblystoma,^ Rana fiisca 

 and esciilenta, and Bombinator,^ rotation ^ig. 114 



of a piece of the gill-area through 180° Ventral view of larva of Boju- 



is followed by development of the gills ^^'"^^^r in which the ectoderm 



. , 1 • A \ • J of the gill-region on the left 



(and operculum m Anura) m reversed gj^e had been rotated through 

 orientation. This shows that the field is 180°. * limit between rotated 

 polarised along an antero-posterior axis. ^^fi.i^roTfLetlS.bT^; ^\ 

 At the same time, the fact that at this op, operculum. (From Braus, 

 early stage it is still only a field and Zeitschr.f.MorpJuu.Anthrop. 



. ,, , • tt 1 XVIII, 1914.) 



not a spatially and regionally deter- 

 mined rudiment is shown by the capacity of two rudiments grafted 

 together to regulate and give rise to a single normal set of gills 

 (provided that the antero-posterior axes coincide)^ (fig. 114). 



Turning now to the heart, it is found in embryos of Bomhinator 

 at the neurula stage that if the presumptive heart-area (which occu- 

 pies a region of the mesoderm) is extirpated, a heart is formed from 



^ Tokura, 1925. 

 ^ Harrison, 192 1 b. 



- Kaan, 1926. 



* Ekman, 1913, 192; 



