1 68 BIOLOGICAL LECTURES. 



off are transformed into a curious mucin and bodily extruded ; 

 this happens also in the sebaceous glands, where they are 

 converted into sebum ; in the oil gland in the tails of birds ; 

 in the skin itself, in which the outer cells are converted into 

 keratin. In fact, nearly all the appendages of the skin, such as 

 hairs, feathers, and nails, are secretions of this sort. It will be 

 seen that it is impossible to draw a line between such simple 

 processes as the budding off of a portion of the cell and the 

 well-characterized cell division. As a general rule, however, 

 only those processes of this kind are properly designated as 

 secretions in which the cells budded off, or given off, do not 

 divide again, but are soon transformed into materials plainly 

 lifeless. 



A third way in which these complex substances are removed 

 from the cell is by a compression of the cell by contractile 

 tissue. The substances are here squeezed out and the cell 

 temporarily disrupted. This is a fairly common method. One 

 of the most striking and elaborate examples is furnished by the 

 nematocysts of the Hydrozoa. In this case the hylogen to be 

 secreted takes the form of a spirally wound thread forming a 

 dart. This secretion is expelled violently from the cell by the 

 contraction of a specific part of the cell's protoplasm around 

 the cyst containing the dart. Here we have a secretion used 

 for defensive and offensive purposes. Other clear examples of 

 secretions of this sort are furnished by the large unicellular 

 glands of the edge of the mantel of Aplysia and the unicellular 

 glands of the carp-louse, Argulus. These are emptied of their 

 watery secretion by means of a special muscular sheath sur- 

 rounding the cells in Aplysia, and apparently by the body 

 musculature in Argulus. Many of the secretions from the 

 multicellular glands are produced in the same way. This is 

 the case in the livers of the moUusks and arthropods, the skin 

 glands of Amphibia, the poison glands of spiders, the sweat 

 glands of mammals, and, as will be pointed out later, the 

 salivary glands of mammals and cephalopods. The muscular 

 movements of the intestinal wall are not improbably a con- 

 siderable factor in most animals in the secretion of the glands 

 and cells lining this tract. 



