Chapter VII 

 THE MOSAIC STAGE OF DIFFERENTIATION 



§1 



It has been seen in previous chapters that, after a certain stage, the 

 various regions of the early embryo are irrevocably determined to 



^^ 



'C 



-^ 







^^^^1^^ 



B 



Fig. 92 

 Mosaic development in explanted tissues. A, Isolated head, containing brain, 

 eyes, nasal pits, tip of notochord, cartilage, and functional jaw-muscles, 

 differentiated in inorganic culture medium from the dorso-anterior portion of 

 an early neurula of Ambly stoma: 25 days after explantation. B, Isolated trunk, 

 containing spinal cord, notochord, muscles, pronephric tubules, gut, fore-limbs, 

 and tail with dorsal fin, differentiated in inorganic culture medium from the 

 postero- ventral portion of an early neurula of Triton : 25 days after explantation. 

 (From Holtfreter, Arch. Entwmech. cxxiv, 193 1.) 



undergo some particular type of development, although at the time 

 that they are thus determined there is no visible differentiation 

 of any kind. This determination is presumably to be ascribed to 

 the local elaboration of specific chemical substances, and may be 



