450 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



tion itself, for the territory nearest to the neuro-pituitary acquires the basophihc 

 cells, i.e. the thyrotropic and the gonadotropic elements, while the eosinophilic 

 cells, which are shown in this work to be corticotropic, appear farther from the 

 neuro-hypophysis, and remain present for the longest time when there is no longer 

 contact with the posterior lobe (Pasteels Jr., 1954, 1957)- It is pertinent that the 

 important endocrine functions of the pituitary owe their existence to a tertiary 

 induction exerted by the medio-ventral part of the young thalamo-encephalon, 

 being itself the hind-median part of the acrencephalon. 



The normal induction of the olfactory placodes has been thought to depend 

 on the forebrain and the endoblast. However, after a re-examination of the problem. 

 Haggis (1956) concludes that the anterior part of the prosencephalon plays the 

 exclusive role. Nevertheless, olfactory placodes have been obtained in the absence 

 of prosencephalon in some of Gallera's experiments (p. 373) ; they were associated 

 with a rhombencephalon, and their presence seems related to the presence of the 

 anterior part of the archenteron roof. 



The causal correlation between retinal layer and lens was the first to be postu- 

 lated in the study of Entwicklungsmechanik, and the first to be experimentally 

 demonstrated. The discovery of this logical link marked the beginning of the 

 finest chapter in the history of developmental biology [cf. Woerdeman, 1955b). The 

 often-quoted difficulty which arose from experimental cases of apparently in- 

 dependent lens formation elicited much research with far-reaching consequences 

 {cf. Ten Gate, 1 956) . The sensibility of serological methods has permitted workers 

 to prove (Ten Gate & Van Doorenmalen, 1950, Flickinger et al., 1955) the exist- 

 ence of organ-specific protein'. Woerdeman (1950, cf. 1955b, p. 43) could even use 

 this opportunity to demonstrate what seems to be chemical induction in vitro. He 

 first tested saline extracts of the head epiblast of young axolotl neurulae before the 

 appearance of lens placodes, and found that they did not react with antiserum to 

 adult lenses. Then he tested extracts of young isolated optic vesicles in the same 

 way, and found them to be also inactive. Finally, he mixed equal parts of both 

 saline extracts, incubated the mixture at 37°G for 24 hours, and got a positive 

 reaction, while the separate extracts incubated for the same length of time re- 

 mained negative. The clear meaning of this remarkable experiment is that the 

 ingredients for the synthesis of the specific lens proteins are located in the two 

 ectoblastic primordia, optic vesicle and epiblast, and simply need to be brought 

 together to produce the normal result of this induction. A relatively similar ex- 

 periment has been performed by De Vincentiis (1954) on the frog; this author 

 obtained a positive precipitine reaction between an anti-lens rabbit serum and a 

 homogenate of lens presumptive epiblast, but only when this extract was mixed 

 with an extract of otocysts. 



The intimate nature of the action exerted by the retinal layer on the epiblast 



' Recently, Van Doorenmalen (1957) has been able, by combining anti-lens sera with a 

 fluorescent dye (isocyanide), to detect a fixation of the antibodies on the germinative zone 

 of the lens. In the same Amsterdam laboratory, Langman (1957) cultivated primary optic 

 vesicles with the lens forming epiblast in the presence of anti-lens sera and obtained definite 

 alterations of the lens area. Other results also suggest that during the period of induction 

 the cells possess free groups which are able to react with the antisera. 



