178 BIOLOGICAL LECTURES. 



isms. But this is a rather limited use of the term "secretion." 

 As we know, there are several glands in the organism whose 

 products of secretion are never emitted to the external world, 

 but put to some definite use within the organism. Discharge 

 of the material from the organism that produced it is not the 

 essential characteristic of the secretion process. 



The process of secretion does not necessarily imply the ex- 

 istence of a gland. When one examines the cytological aspect 

 of the phenomena of secretion, it resolves itself into this : dis- 

 integration of the protoplasm and the subsequent conversion 

 of the material thus produced into the secretion granules, or 

 the separation of the specific substance from the more or less 

 crude material incorporated within, or brought in contact with 

 the body of the cell, under the influence of the protoplasm. 

 So far as the fundamental process of secretion is concerned, 

 it may be carried out by an isolated single cell just as well as 

 by the hundreds and thousands of similarly constituted gland 

 cells forming a sac-like recess, opening on the free surface 

 of the organ or of the organism. 



In the case of ordinary, secretion, such' as saliva, the granules 

 formed by the metabolic activity of the protoplasm undergo 

 still further changes, and eventually escape from the body of 

 the cell. In other cases these granules remain as such, and 

 subsequently become consumed by the activity of the proto- 

 plasm that produced them, as is well seen in the history of the 

 yolk-granules in the egg-cell. The granules produced by the 

 process of secretion at one period become utilized by the 

 growing egg-cell in the course of its own development. 1 



Just as the liver or the thyroid is an organ of " internal 



1 " The egg-cell of most animals, at any rate during the period of growth, is by 

 no means an indifferent cell of the most primitive type. At such a period its cell- 

 body has to perform quite particular and specific functions ; it has to secrete nutri- 

 tive substances of a certain chemical nature and physical constitution, and to store 

 up this food material in such a manner that it may be at the disposal of the embryo 

 during its development. In most cases the egg-cell also forms membranes which 

 are often characteristic of particular species of animals. The growing egg-cell is, 

 therefore, histologically differentiated, and in this respect resembles a somatic cell. 

 It may perhaps be compared to a gland cell, which does not expel its secretion, but 

 deposits it -within its own substance." Weismann, The Continuity of the Germ- 

 plasm as the Foundation of a Theory of Heredity, 1885. 



