. The biology of the frog . sms are united to form tissues whichare aggregations of cells of similar character bound togetherby means of an intercellular substance. In the bodies ofanimals the classes of tissues commonly distinguished arethe following : — 1. Epithelial. 2. Connective. 3. Muscular. 4. Nervous. These broad divisions include nearly all the manifoldvariety of cells occurring in the body. The blood andlymph are sometimes added as forming a distinct class oftissues, sometimes classed as a form of connective tissuewith fluid intercellular substance, and sometimes treated ofas if they
. The biology of the frog . sms are united to form tissues whichare aggregations of cells of similar character bound togetherby means of an intercellular substance. In the bodies ofanimals the classes of tissues commonly distinguished arethe following : — 1. Epithelial. 2. Connective. 3. Muscular. 4. Nervous. These broad divisions include nearly all the manifoldvariety of cells occurring in the body. The blood andlymph are sometimes added as forming a distinct class oftissues, sometimes classed as a form of connective tissuewith fluid intercellular substance, and sometimes treated ofas if they were not tissues at all. They will be describedin a later chapter. In the epithelial tissues the cells lie in layers with only asmall amount of intercellular substance. We meet with thisclass of tissue on the surfaces of organs, or lining the cavi-ties of organs, and forming the lining of glands, bloodvessels, and ducts of all kinds. The various kinds of epithe-lium are distinguished according to the shapes of the 122 THE BIOLOGY OF THE FROG chap. An excellent example of flattened or squamous epitheliummay be obtained in the outermost skin which is cast offduring the molt. The cells of this layer are broad and ex-ceedingly thin, and show a roundednucleus near the center. The cellsof the peritoneum are mostly of thesame flattened type. In the colum-nar epithelium the cells are elon-gated perpendicularly to the sur-face and are usually prismatic inoutline, owing to mutual pressure;such epithelium is common in thefig. 31. —a portion of the mucous layer of the intestine. Inepidermis of Ranapipiens. many piaces, as in the outer skin, s, stoma cell. . „ ... there may be all transitional stagesbetween columnar epithelium and squamous such as this which are several cells deep are calledstratified epithelium. In some parts of the body there occurs a peculiar varietycalled ciliated epithelium in which the cells are furnishedwith cilia at their outer ends.
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