The Delayed Development of the Stomach in the Frog (<i>Rana temporaria</i>) and the Toad (<i>Bufo bufo</i>).

Abstract

S ummary An account is given of the structure of the anterior end of the alimentary canal of the frog and toad tadpole. It is shown that there lies between the pharynx and the main intestine (mid‐gut) a clearly defined fore‐gut, characterized by the possession of an epithelium composed of ciliated and mucous cells which are distributed according to a definite pattern. At metamorphosis the oesophagus and stomach differentiate out of the fore‐gut, and the presence in the latter of the ciliated and mucous cells clearly foreshadows this fate, since cilia are found in the oesophagus of the adult, and mucous cells characterize both this region and the stomach. It is further possible that two types of mucous cell visible in the fore‐gut may foreshadow the goblet cells of the future oesophagus and the gastric epithelial cells of the future stomach, but this is less certain. The so‐called “stomach” of the frog tadpole lies in the fore‐gut region and possesses gland‐like structures which, however, differ histologically from those of the adult stomach. Tests for pepsin and amylase have given negative results, and it is doubtful whether the organ is zymogenous. It is altogether absent from the toad. Pepsin first becomes identifiable in the fore‐gut region of the frog and toad after the emergence of the anterior limbs, at the time when the definitive glands are forming. Thus gastric digestion does not occur in the larva, and the structure of the epithelium indicates that this may be ascribed to a retardation in the rate of differentiation of the oesophagus and stomach. Possible reasons for this are discussed in the light of the reported occurrence of true gastric digestion in urodele larvae, and of the differences in feeding habits of the larvae of the two groups.