On the Nature of Head Malformations Obtained by Centrifuging the Eggs of Limnaea stagnalis Available to Purchase

Eggs of Limnaea stagnalis were centrifuged at a late 4-cell stage for 10 minutes at a centrifugal pressure of 925 × gravity. The head malformations produced by this treatment were studied.

No cyclocephalic malformations were obtained. Apparently centrifuging does not produce a depression of the animal gradient-field.

No median reduplications with a splitting of the apical plate were found. Hence centrifuging does not cause a strengthening of the animal gradient-field either.

Lateral reductions of eyes, tentacles, and cerebral ganglia were obtained in a large percentage of the cases. They were equally distributed between both sides of the head. The frequency of lateral reductions decreases in the order: eye, tentacle, cerebral ganglion.

Lateral eye reduplications occurred in a certain number of cases. They were not accompanied by a reduction of the velum.

A complete splitting of the foot in two lateral halves was found in one case.

The most characteristic effect of centrifugation consists of the formation of a supernumerary cerebral plate in an abnormal place. This may lie wholly in the post-trochal or in the pre-trochal region, or on the boundary between the two.

As the supernumerary cerebral plate approaches one of the ‘host’ cerebral plates, the differentiation of the latter is more and more suppressed. Finally, the supernumerary plate may almost altogether supersede and replace one of the normal cerebral plates.

The formation of a supernumerary cerebral plate cannot be explained by a disturbance of the animal gradient-field. The supernumerary plate appears to be superimposed upon a relatively undisturbed pattern of head organs.

It is concluded that the normal pattern of head organs is controlled by a gradient-field located at early cleavage stages in the relatively immovable cortical layer. The differentiation of the cerebral plate, on the other hand, is dependent on a morphogenic substance which may be displaced by centrifugal force.

In normal development, some kind of interaction between morphogenic substance and gradient-field takes place. The most simple hypothesis appears to be that the localization of the ‘cerebral plate substance’ during ooplasmic segregation is controlled by the cortical gradient-field.