392 



HISTOLOGY 



cuticle that is found opposite each gland-cell. It is used not only to 

 lubricate the body but to permanently line the tube in which the animal 

 lives with a mucous covering. 



These cells are, in some stages, very much like a number of the other 

 epidermal cells in the epithelium of the earthworm that are also modi- 

 fied for the purpose of secretion. This other kind of cell, however, pro- 

 duces a very different kind of secretion, and it might well be studied as a 

 type of the so-called serous cell or albumen cells of the animal tissues. 

 It differs in the earthworm from the mucin cells in several ways. Most 

 important is the staining reaction of the secretion. This and chemical 

 tests show that it is a different substance from mucin although it is pos- 

 sible, considering the derivation of the two cells and the similarity of the 

 manner in which they secrete, that both cells and secretions were derived 

 from a common kind or that one was derived from the other. 



The simple, unicellular, mucous gland may thus be followed through 

 a series of forms of increasing specialization terminating with the extreme 

 form found in the leech which is discussed in another connection (Chapter 

 XXII). This single mucous cell is found, in some very rare cases, in an 

 epithelium that has become stratified, and Figure 356 shows a case of 

 this kind in a section of part of the alligator's conjunctiva. Here a 



single mucous cell is to be seen 

 rising from the basement mem- 

 brane through the several stratified 

 layers, and opening freely to dis- 

 charge its secretions. Similar con- 

 ditions are found in the skin of 

 many fishes. 



The mucous cell is found not only 

 in the single-celled forms dealt with 

 in the above paragraphs, but also 

 collected into groups that are mostly 

 invaginated into glands of varying 

 complexity. Such a gland may be 

 found to be derived from, and open- 

 ing on to, a simple epithelium or a 

 stratified epithelium. The former 

 should be considered a more primi- 

 tive form, perhaps, and can be easily 

 seen in the mucous secreting glands 

 which open into the small intestine. 

 These are known as the duodenal 



glands or Brunner's glands, and the simple epithelium which lines them 

 can be traced through their ducts and into the simple columnar epithe- 





FIG. 356. Vertical section through a bit of 

 the stratified epithelium lining the alli- 

 gator's conjunctiva. Shows one mucous 

 cell extending through the epithelium. 

 X 880. 



