Induction of the Olfactory Placode by the Forebrain in Rana pipiens

Bell, 1 using embryos of Rana esculenta, attempted to show the relation of the brain to the development of the olfactory placode. He transplanted “that part of the brain which forms the olfactory lobe” to other regions of the body without obtaining any induction of an olfactory placode. His conclusion was that the olfactory placode developed independently of the brain. He did not, however, preclude the possibility of an early stimulus for olfactory placode formation being imparted by the brain to the ectoderm above it.

Recently we have been able to show that the part of the fore-brain median and slightly anterior to the optic vesicle of the intermediate neurula stage of Rana pipiens has the power to induce an olfactory placode in the ectoderm of the flank (Fig. 1).

After one accidental induction (from a transplant of an optic vesicle which evidently included some of the rest of the fore-brain) 3 series of operations were performed in an attempt to localize the olfactory placode inducing part of the brain. The following stages were used for the operation:

Early neurula—equivalent to stage 17 on Harrison's scale for Ambystoma punctatum.

Intermediate neurula—equivalent to stage 20 on Harrison's scaie for Ambystoma punctatum.

Late neurula—equivalent to stage 22 on Harrison's scale for Ambystoma punctatum.

Early tail-bud—in which the olfactory placode is visible as a pigmented thickening of the ectoderm.