THE MOSAIC STAGE OF DIFFERENTIATION 235 



Stage, a piece of the heart-field can be rotated through i8o° and 

 still give a normal heart, but this is no longer possible at later stages 

 such as the tail-bud. ^ As in the case of the limb, ear, and gill-fields, 

 therefore, the heart-field is polarised along an antero-posterior axis 

 from an early stage. 



A normal heart can be formed from a longitudinal half-rudi- 

 ment ; a single rudiment, split lengthwise, can be made to give rise 

 to two or even three hearts ; and two rudiments grafted together at 



^f 



^--^ .#:.. 



A B 



Fig. 116 

 Power of regulation of the heart-field, in Bombinator. At the neurula stage, the 

 left half of the heart-field was extirpated; the right half, remaining in situ, has 

 regulated to form a complete heart, with sinus venosus {S) ; atrium {A), ventricle 

 (F), and bulbus {B). The histological differentiation of the parts is normal. 

 Certain details of morphological differentiation are, however, abnorrnal; the 

 ventricle projects to the left, and its long axis may be longitudinal (as in A) or 

 transverse (as in B). Figures taken 14 days after operation. (From Stohr, Arch. 

 Entwmech. cxii, 1927.) 



the neurula stage can regulate to form one normal heart, provided 

 that both the antero-posterior axes are similarly oriented.'^ 



All these results have been confirmed for Urodela by experiments 

 on Amblystoma.^ 



The epidermis itself may be regarded as a large field, the deter- 

 mination of which is characterised not so much by any positive 

 differentiation (for this is comparatively slight), but by the pro- 

 gressive incapacity to differentiate mto other structures, e.g. lens. 

 Nevertheless, the epidermis possesses a polarity, and this is ex- 



1 Stohr, 1925. - Ekman, 1924. 



^ Copenhaver, 1926. 



