266 



Embryogenesis: Progressive Differentiation 



Fig. 99. 



# 





:-m 





^m 





Fig. 100. 



Fig. 99. Induction of neural structures and eye with lens, obtained by implantation of heat-killed anterior 

 medullary plate into the blastocoele of an early gastrula of Triturus (from Holtfreter, '35e). 



Fig. 100. Induction of a secondary embryo obtained by implantation of a piece of boiled human thyroid 

 gland into the blastocoele of an early gastrula of Triturus. Notice neural tube, notochord and otocysts in b. 

 (From Holtfreter, '34b.) 



two experimental modifications (Holtfreter, 

 '33e): (1) the heat-killed tissues were sand- 

 wiched between two sheets of isolated pro- 

 spective epidermis from an early gastrula; 

 (2) pieces of prospective epidermis were 

 loosely placed upon the dead tissues. In both 

 cases, voluminovis inductions were obtained 

 in the explants representing brainlike forma- 



tions (Fig. 101) which were sometimes ac- 

 companied by mesenchyme, melanophores, 

 or an olfactory placode. 



Graded inactivation of neural inductors by 

 heat was as a rule expressed in three inter- 

 related phenomena: relative frequency, vol- 

 ume, and organological complexity of the 

 induced structures. If large enough, the in- 



