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The height of the apical ectodermal ridge on limb buds of the embryo laboratory rat was studied in the polydactyly-luxate syndrome and compared with the controls. The following findings were obtained: (a) On the 14th embryonal day, prior to the development of the anlage of mesenchymal condensates, the AER is higher in polydactylous animals as compared with the controls. (b) On the 15th, 16th and 17th embryonal day the height of the AER in the praeaxial region of the polydactylous limb bud largely predominates over the controls. A comparison of the height of the AER above digital rays and interdigital grooves of polydactylous and normodactylous animals does not thus exhibit any marked differences. This fact is attributed to the existence of more powerful induction processes of the underlying mesenchymal component where rudiments of supernumerary digital rays are formed.

In hindlimb buds of normodactylous and polydactylous embryos in the stage of the 16th and 17th embryonal day the mesenchymal region, closely adjoining the site below the AER, was investigated. This space is called the subridge zone and is filled with a large amount of variously formed processes of mesenchymal cells, chiefly with filopodia. With the use of the morphometric point-couting method it was found that in normodactylous limb buds the density of filopodia in the given area of the subridge zone was 2.15% as compared with the 6.48% representation of filopodia in the same zone of polydactylous animals. Numerous filopodia localized right under the AER established connection between the mesenchyma and the intact basal membrane, and their higher density is undoubtedly related to the existence of the maintenance factor.