Ill 



SECONDARY AND MINOR INDUCTIONS 



451 



has been frequently investigated (see Stone and Dinnean, 1943) and the necessity 

 of an intimate contact with the reacting epiblast much discussed (Weiss, 1949a, b; 

 Mc Keehan, 1951), without reaching a final conclusion. From culture and trans- 

 plantation experiments, it has appeared that lens induction is not a trigger-like 

 process, but a continuous, gradual one (Mc Keehan, 1954; Liedke, 1955). A 

 precise quantitative study by Mc Keehan (1956) has compared the variation of 

 RNA content in the retinal layer and the lens of chicken optic vesicles. He con- 

 cludes that the nearest interpretation is that the retinal RNA is broken up into 

 smaller molecules and transmitted to the lens, a process already admitted by Gallera 

 and Oprecht (1948). It is however likely that these cytochemical modifications 

 are only an expression of phenomena concerning the ribonucleoproteins. The 

 factors determining the size of the lens have been examined by several authors, 

 especially by Balinsky (1957). 



Fig. 92. Modification of the retinal layer of a rat optic vesicle when implanted in the anterior 

 eye chamber of an adult, (a) The implant was a foetal optic vesicle (i i 1/2 days) still deprived 

 of lens, and liberated of its mesenchyme ; after 6 days in the host, it has become uniformly 

 retinal; left, the host iris; (b) implant was a vesicle with a lens (12 1/2 days), with most of 

 its mesenchyme pealed off; no pigmented layer has differentiated, except in a small zone 

 of contact with the mesenchyme (above) ; right, the host cornea; mes. - mesenchyme; p.l. - 

 pigment layer; p.e. - posterior epithelium of the cornea. From Stroeva, 1956b. 



The distinction between the retinal pigmented layers of the optic vesicle has 

 been known for a long time to be experimentally modifiable. An overproduction 

 of retinal parts has been obtained in Ambystoma by grafting the vesicle into contact 

 with the otocyst (Detwiler and Van Dyke, 1953; Detwiler 1954b). The most effi- 

 cient part of the otocyst appeared to be the region of the lateral canal with its crista, 

 and the macula utriculi. Let us remark that these active regions of the epithelium 

 are also quite rich in RNA. Stroeva (1956a) reports that a partial transformation 

 of the pigmented layer into retinal islands is obtainable by rearing frog tadpoles 



Literature p. 483 



