EMBRYONIC INDUCTORS AND ORGANIZERS 481 



by chorda-mesoderm or any other part. After removal of entoderm there 

 is apparently some reconstitution of entoderm, probably from ectoderm; 

 and entoderm may perhaps have some inductive action on the papillae of 

 attachment. Rose's data suggest induction at a very early stage, but how 

 far it is similar to amphibian induction is not yet evident; conceivably, 

 it may have taken place before the stage of von Ubisch's experiments. 



FISHES 



The posterior part of the teleost embryonic shield, representing the 

 dorsal lip of the blastopore, possesses inducing capacity, as in amphibians. 

 When pieces of it are transplanted to other regions, even the extraembry- 



A B 



Fig. 161, A, B. — Induction in the teleost, Fundulus. A, induced embryo (/) in primary 

 embryonic shield; B, induced embryo (/) outside primary shield (from Oppenheimer, 1936c). 



onic regions of the blastoderm, they may induce secondary embryos. ^^ 

 When transplantation is within the embryonic shield, the induced and the 

 primary embryo are closely similar, and corresponding parts are at the 

 same levels; but embryos induced by transplants to extraembryonic re- 

 gions show no definite relations to the primary embryo, and corresponding 

 parts may develop at different levels (Fig. 161). The polarity of the em- 

 bryo induced in the extraembryonic region may even be opposed to the 

 host polarity. Evidently, within the embryonic shield the longitudinal 

 pattern of the host determines direction of the induced secondary em- 

 bryonic axis and the levels at which its organs develop. In extraembryonic 

 regions gradient pattern is slight or absent; consequently, the inductor 

 gradient may become the chief or only factor in determining axial direc- 

 tion. However, blastomeres explanted to salt solution may undergo more 



■i- Oppenheimer, 1934, 1936a, b, c; W. Luther, 1935, 1936a, h. 



