Experimental Study of Factors Inhibiting Differentiation of Proctodeum

Introduction. It was previously reported (Schechtman 1 ) that the ventrolateral blastoporal lip of the early gastrula (Hyla regilla) is capable of inducing a proctodeum in the belly epidermis of another embryo. The percentage of positive results was extremely variable : occasionally a group of 6 host embryos might develop an accessory proctodeum in all 6 cases; however, in most groups of 6, positive results occurred in only 1, 2, or 3 cases. This variability persisted in over 200 implantations carried out during the laying seasons of 3 consecutive years. In the entire group slightly more than 50% showed clear accessory proctodea. Accessory proctodea and tails were also produced by centrifuging late blastulae and early gastrulae (Schechtman 2 ). An analysis of 983 such embryos by one of us (Cannon, unpublished) showed that only 4% were clearly positive. A striking characteristic of both implanted and centrifuged eggs is that accessory proctodea, with rare exceptions, develop on the belly region.

The aim of the present investigation was to determine the causes behind the great variability shown by the ventral blastoporal lip in its capacity to induce proctodea. The observed fact that the induced proctodea are almost always in the belly region-never adjacent to or over the axial organs-suggested that the latter may inhibit the development of proctodea.

Experiments. In the transplantation experiments mentioned above, the orientation of the proctodeum-inductor could not readily be controlled since the material was placed in the blastocoele. In the present experiments the proctodeum-inductor was vitally-stained and placed in a superficial position in the presumptive ectoderm of early gastrulae, so that its orientation was never in question. The experiments were as follows:

(1) Two vitally stained dorsal lips (which contain presumptive chorda-somite material) were transplanted, one on either side of the ventral lip of a host. The implanted dorsal lips invaginated well, so that 2 separate accessory medullary plates developed in addition to the host's own medullary plate.