. The biology of the frog . ubmucosa extends into the folds of the innercoat. Between the mucosa and submucosa there is a thinmuscular layer, the muscularis mucosa, composed of aninner layer of circular fibers and an outer stratum of longi-tudinal ones. The mucosa of the stomach is a thick layer composed ofglands embedded in a supporting matrix of connective glands represent invagination of the epithelium liningthe inner surface of the stomach. They are elongatedtubular structures set very closely together,-and frequently 140 THE BIOLOGY OF THE FROG more or less branched. The glan
. The biology of the frog . ubmucosa extends into the folds of the innercoat. Between the mucosa and submucosa there is a thinmuscular layer, the muscularis mucosa, composed of aninner layer of circular fibers and an outer stratum of longi-tudinal ones. The mucosa of the stomach is a thick layer composed ofglands embedded in a supporting matrix of connective glands represent invagination of the epithelium liningthe inner surface of the stomach. They are elongatedtubular structures set very closely together,-and frequently 140 THE BIOLOGY OF THE FROG more or less branched. The glands differ in structure atthe two ends of the stomach. In the cardiac region theglands are very long, the mouth of the gland is quite deep,and lined with elongated cells whose clear inner endsare filled with a substance which probably forms the outer end of the gland the cells are more elongated,like those of the surface epithelium: behind the clear sub-stance the cytoplasm of the cells is granular, the nucleus is. Fig. 39. •Glands of the stomach. A, from cardiac end; B, from pyloricend ; m, mouth ; n, neck ; b, body of gland. elongated, and the outer ends are drawn out into a longnarrow process. Passing down the mouth of the gland thecells become shorter, the nuclei more rounded, and the tail-like processes finally almost disappear. In the neck regionsof the gland there are usually a few rather Targe cells con-taining a large clear vacuole which pushes the nucleus andmost of the cytoplasm to one side. It is usually in theregion of these clear cells that the glands branch. The cellscomposing the body of the gland lie just below the clear vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 141 cells and present a very different appearance from the cellslining the mouth and neck. They are polygonal in outline,with large round or oval nuclei and granular cytoplasm ; thelumen or central cavity of the gland in this region is verysmall and at times almost obliterated. The lower en
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