EMBRYONIC INDUCTORS AND ORGANIZERS 487 



tain. In various hydroids hydranths degenerate and stolons develop, even 

 from apical, as well as from basal, ends, in standing water, in KCN, and 

 with other inhibiting agents, while in flowing, well-aerated water hy- 

 dranths develop (pp. 172-75). Is this fundamentally different from devel- 

 opment of presumptive amphibian epidermis as epidermis under certain 

 conditions, as neural tissue, or as mesoderm under others? 



LENS INDUCTION 



Among the organ inductions of later embryonic stages that of the lens 

 by the optic cup in amphibians was known before induction of neural 

 plate by chorda-mesoderm and has been the subject of much experiment 

 and discussion. ■5'' 



DEVELOPMENT OF THE AMPHIBIAN LENS 



In the anuran map (Fig. 150) the presumptive lens region is at some 

 distance from the region of the optic primordium, but in the course of 

 development the optic cup comes to lie beneath the presumptive lens 

 ectoderm. Under natural conditions the lens develops from the epidermis 

 as the optic cup comes into contact with it. The amphibian optic vesicle 

 arises as a hollow lateral outgrowth from the developing brain and re- 

 mains attached to it by a stalk. Its wall, at first with little regional differ- 

 ence in thickness, becomes, in part, the thick retinal layer, in part the 

 much thinner tapetum ; the retinal layer becomes concave lateroventrally, 

 forming the optic cup, and lens formation occurs by a proliferation and 

 thickening of the inner layer of the epidermis overlying the concavity of 

 the optic cup. As the concavity becomes deeper, the developing lens ex- 

 tends farther into it and separates from the epidermis. Earher stages of 

 lens formation are indicated in Figure 162. This course of development 

 very naturally suggests that the optic cup may have something to do 

 with lens development. This is also indicated by the fact that, in cases 

 of approximation of eyes and cyclopia, lenses develop in normal position 

 with respect to the optic cup, but presumably from other epidermis than 

 the presumptive lens-forming region. Decisive evidence concerning the 

 role of the optic cup in lens formation has been sought by various lines 

 of experiment: replacement of presumptive lens epidermis by other epi- 

 dermis, removal of optic vesicle preceding lens formation, implantation 

 of optic primordium in other regions, etc. 



•5* For fuller discussion of the data than is possible here and for the literature see O. Man- 

 gold, 193 i(x; Spemann, 1936, 1938. 



