Experiments on Anus Formation in a Frog Egg

Abstract

The anus arises in Hyla regilla, as in most vertebrates, as an ectodermal (proctodeal) pit which subsequently becomes perforated to form the anus. We have attempted to analyze the factors responsible for this differentiation.

The presumptive proctodeal ectoderm of the early gastrula has no capacity to form a proctodeal pit. This ectoderm was transplanted to the chest region, where it formed smooth epidermis. The anterior ectoderm (of the face and chest) were transplanted to a posterior position, just over the ventral blastoporal lip, and here developed into very clear proctodea. Evidently the ventral lip contains some factor or factors responsible for the appearance of the proctodeum, as was previously concluded. 1 When the ventral lip is implanted into the blastocoele of another early gastrula, the host ectoderm develops a proctodeum usually on the side or belly.

In 2 series of extirpation experiments the mesoderm or the entoderm of the ventral lip was removed from the early gastrula. In the absence of the entoderm a proctodeum forms in all cases. After removal of the mesoderm most of the eggs fail to form a proctodeum, but a few develop a complete or rudimentary proctodeal pit. This we attribute to incomplete removal of the mesoderm.

Ventral lip mesoderm from the early neurula, in which the 3 germ layers are more clearly distinguishable, was implanted into the blastocoeles of early gastrulae. The mesoderm, taken from the region just under the slit-shaped blastopore, induces a clear proctodeum in the host ectoderm. The entoderm of the hind-gut region was implanted into other early gastrulae and was not capable of inducing a proctodeum. But the epidermis of the host lying just over the implant becomes perforated, exposing the implanted piece of hindgut.