36 



GENERAL PART. 



however, it is generally difficult to distinguish from certain connective 

 tissues. 



While 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 

 importance from a chemical point of view. In this case the epithe- 

 lium becomes 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 



FI&. 24. Gastric glands, a, their origin, as in- 

 vaginations of the epithelium, b, perfect gas- 

 tric glands. 



it (tig. 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 secretion. The gland 

 then has the form of a sack or 

 blind tube, derived from an inva- 

 ginatioii 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 regular and 

 irregular outgrowth. While their form presents great variations, 

 they are universally characterised by the transformation of their 

 terminal portion into a duct ; this differentiation may also appear 

 in the simple glandular tubes, and even in the unicellular glands 

 (figs. 23, 24). 



FIG. 23. Unicellular glands, a, goblet 

 cells from the epithelium of the small 

 intestine of a vertebrate, b, unicel- 

 lular cutaneous gland of Argulus 

 with long duct, c, unicellular cuta- 

 neous gland of insects with cuticular 

 duct. 



