Volume 1

Elementary text-book of zoology / by C. Claus ; translated and edited by Adam Sedgwick ; with the assistance of F.G. Heathcote.

  • Carl Friedrich Wilhelm Claus
Date:
1889-1891
Catalogue details

Licence: Public Domain Mark

Credit: Elementary text-book of zoology / by C. Claus ; translated and edited by Adam Sedgwick ; with the assistance of F.G. Heathcote. Source: Wellcome Collection.

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    however, it is generally difficulfc to distinguish from certain eonnective tissues. Wliile cuticular structures are solid secretions which are of use in supporting and giving a definite form to the organism, there are, on the other hand, various fluid secretions proceeding from cells which give rise to no structures, and which are often of considerable importänce from a Chemical point of view. In this case the epithe- lium hecomes glandular tissue. In the simple cases the gland is constituted of a single cell, the secretion of which either passes out through the free surface of the membrane, or a special opening in Fig. 23.—Uuicellular glands. a, goblet cells from the epithelium of the small intestiue of a vertebrate. b, unicel- lular cutaneous gland of Argulus with long duot. c, uuicellular cuta- neous gland of insects with cuticular Fig. 24.—Gastric glands. a, their origin as in vaginations of the epithelium. b, perfect gas- tric glands. it (fig. 23). If several cells enter into the formation of a gland, they are arranged, in the simplest cases, round a central cavity, which receives the seci'etion. The gland then has the form of a sack or blind tube, derived from an inva- dnct. gination of the epithelium, either of the inner or the outer surface of the body, into the subjacent tissue. From this fundamental form the larger and more complicated glands are to be derived, as the result of continued regulär and irregulär outgrowth. While their form presents great variations, they are universally characterised by the transformation of their terminal portion into a duct; this difterentiation may also appear in the simple glandular tubes, and even in the unicellular glands- (figs. 23, 24).
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