THE MOSAIC STAGE OF DIFFERENTIATION 



203 



in a normal rat of similar age.^ Nasal sacs and mesonephros 

 achieve a less but considerable degree of differentiation. 



Explantation methods have also been applied to the problem. 

 Presumptive rudiments of organs, as yet without any visible differ- 

 entiation, are removed from the body and allowed to develop in 

 culture media. In some cases they are enclosed within jackets of 

 epidermis, but this is not an essential condition. In addition to 



Fig. 98 



Self-differentiation of median heart- 

 rudiments in vitro. The heart-rudi- 

 ments together with some of the 

 neighbouring ento-mesoderm were 

 removed from early tail-bud stages 

 of Bornhinator , and cultivated as ex- 

 plants in epidermal jackets. Above, 

 part of a micro-cinema film of an 

 explanted heart, 11 days after opera- 

 tion; a. systole; ^.diastole. Below, 

 longitudinal section through a similar 

 explant showing differentiation into 

 a auricle; v, ventricle. In addition, 

 /, liver; d, yolk-sac; £), gut, also pre- 

 sent in explant. (From Stohr, Arch. 

 Mikr. Anat. u. Entwmech. cii, 1924.) 



notochord, neural tube, muscle-segments, epidermis and kidney 

 tubes, 2 auditory vesicles, gut, liver, and pancreas-rudiments of 

 amphibian embryos treated in this manner develop for considerable 

 periods of time, and produce their appropriate structures, including 

 functional ciliated epithelium and secretory tissue with actual secre- 

 tion : portions of gut thus differentiated may even show peristaltic 

 action.^ 

 Paired heart-rudiments of Urodele embryos at the neurula stage, 



Hiraiwa, 1927; Nicholas and Rudnick, 193 1, 



2 Erdmann, 1931. 



Holtfreter, 1931A, b. 



