The third division of the oviduct, the _isthmus_, continues from the albumin-secreting portion and terminates in the expanded portion called by some anatomists, the uterus. Toward the posterior end of the albumin-secreting portion the longitudinal folds of mucous membrane become lower, making, at the juncture of this and the isthmus, a clear fine of demarcation. For a distance of 2 or 3 centimeters the folds are low, after which they gradually become higher, but never reach the height or thickness of those in the albumin-secreting portion. The clear-cut line between the albumin-secreting portion and the isthmus is partly due to a zone in which the long tubular glands are lacking. The core of the folds of mucous membrane in this zone contain much more connective tissue. The cells are both ciliated and unicellular. The rest of the histological structure of the isthmus is the same as that of the albumin-secreting portion. The function of the isthmus is to secrete, or to form, the shell membrane, the membrana testacea. The fourth division of the oviduct is the _uterus_, or the shell-gland portion (Fig. 60, _C_). There is no clear line of demarcation between the isthmus and shell-gland portion, the walls gradually expanding. In this region the folds of mucous membrane become leaf-like and of considerable length, extending into the lumen, thus affording a greater cellular surface. The same coats of the duct are present here as in other parts; but the outer longitudinal muscular layer is thicker and possesses more strength. The quantity of connective tissue is about the same. The glands are of a tubular type and the same two varieties of epithelial cells are found here as elsewhere, namely, the ciliated high columnar and the unicellular mucous variety. In the active glandular cells of the shell-forming region the nuclei are small, dark staining, and lie toward the center of the cell. The chromatin granules and nucleoli take on a comparatively deep basic stain, but they do not show the intense stain found in the albumin and the isthmus region. The cytoplasm of the uterine tubular glands does not present as heavy a granular appearance as that of the albumin portion. These cells are diffusely granular, the granules appearing of one size and taking the stain faintly. The function of this portion is to secrete, or form, the hard calcareous covering which has been described at the beginning of this section.