in SECONDARY AND MINOR INDUCTIONS 449 



(a) The cephalic placodes 



The head epiblast produces major and mhior placodes characterized by their 

 topography and their individual features. Two pairs of macroplacodes and a 

 single one are typical of the acromeritic region, while the chordal part acquires 

 one pair of macroplacodes and a host of microplacodes. 



The diagnosis of an acrencephalon could be doubtful if the hind part of it was 

 not soon flanked by the lenses, and the forepart completed by olfactory placodes. 

 Their localization is caused respectively by the optic rudiments and the telen- 

 cephalon, which thus appears primarily as the olfactory part of the brain. The 

 lens first buds from the epiblast, then detaches itself, its cells elongating moderately 

 without building up fibrillae, their close packing within a tough hyaluronic 

 membrane excluding any vascularization. The olfactory placode behaves more like 

 a small bit of neural plate which would remain incorporated in the epiblast, for 

 its cells elongate centripetally in fibers which characterize them as neurones. 



The early specialization of both structures immediately suggests that their 

 inductive agent cannot be identical. Contrary to what happens with homoeogenic 

 induction of which, for exemple, the spinal cord is capable, the acrogenic agent 

 undergoes some transformation when it is incorporated in the optic expansions 

 or in the telencephalic lobes. However, the kinship remains close, for some xeno- 

 inductors have been found to elicit both isolated nasal placodes and lenses while 

 others specifically induce the one or the other structure. It seems that some "de- 

 gradation" of the acrogenic agent gives this result^ On the other hand, the prob- 

 lem can be raised as to whether or not the agents inducing the whole optic vesicle, 

 and the one produced by this organ to induce the lens are identical, related, or 

 diflferent. A result of Sasaki, Kawakami, Mifune and Tamanoi (1957) strongly 

 speaks in favor of an intimate kinship. A crude protein extract from rabbit bone 

 marrow induces brain with optic vesicles from Hynobius gastrular ectoblast, while, 

 applied to epiblast of the caudal bud stage, the same agent evokes a lens. 



The one odd placode forms the anterior lobe of the hypophysis, and was thought 

 to depend on a conjugated induction by the prosencephalon and the pharyngeal 

 endoblast. But, according to Eyal-Giladi (1958) its main inductor would be the 

 anterior part of the notochord (see also Etkin, 1958) acting on the stomodoeal part 

 of the ectoblast, thus on an epithelium already induced by the pharyngeal endoblast. 



The neural lobe may form independently of the adeno-hypophysis (Eakin and 

 Bush, 1957), but it exerts a definite secondary induction on the anterior lobe. In 

 Anurans, it has been seen, by cultivating embryos in sucrose solutions, that the 

 intermediate part does not differentiate in the absence of contact with the neural 

 lobe (DriscoU and Eakin, 1955). In Pleurodeles larvae, the bud of cells proceeding 

 from the placode only acquires the characteristics of the various groups of the ante- 

 rior pituitary lobe if a sufficient contact with the pituitary part of the infundibulum 

 is established. If the neuro-hypophysis is removed or displaced before the first 

 tokens of diflferentiation appear in the anterior part, it becomes assimilated in the 

 pharyngeal epithelium to form mucous cells or even neurogemmas (Pasteels Jr., 

 1954, 1957). Moreover, inductive relationships seem to operate in the difTerentia- 



^ The problem of free lenses was also examined by Okada (1949). 



Liierature p. 483 



