THE GENERAL CONDITIONS OF LIFE 



333 



cell. In many cases the mucus that appears contains genuine mucin, 

 in others it consists of mucinoid substances, but it is always a 

 compound of proteid with some kind of carbohydrate. 1 It is seen, 

 therefore, that in mucous metamorphosis the origin of the mucus 

 lies in the proteid. Mucous metamorphosis occurs also normally 

 in the healthy body, especially in the cells of the mucous membranes 

 of the respiratory and intestinal tracts, as well as of the urogenital 

 system. In the formation of mucus by these mucous cells 

 under normal conditions the whole cell never perishes, but a part 

 only of its protoplasm is transformed into mucus. Almost always 

 mucous cells are cylindrical cells ; their basal part contains the 

 nucleus, and their upper end bounds the free surface of the 

 mucous membrane. It is always the upper, free end of the cell- 



FIG. 146. Mucous cells. A, Three isolated mucous cells ; B, seven mucous cells united. The three 

 at the left are full, the four at the right are empty. (After Schiefferdecker.) 



body, the protoplasm of which is transformed into mucus, swelling 

 up into a transparent mass containing separate protoplasmic 

 granules ; each mass, having no boundary, unites with the mucous 

 masses of the neighbouring cells into a coherent mucous cover- 

 ing. The process is a continual one and is increased by certain 

 external influences. The lower part of the cell-body containing 

 the nucleus continues to live (Fig. 146), and constantly shoves up- 

 ward new masses of mucus-forming substance, or mucigen, which 

 become transformed into mucus in proportion as they move along. 

 A complete transformation of the whole cell-body into mucus, 

 accompanied by the death of the cell, occurs in many lower animals 

 upon strong external stimulation ; the phenomena of this process 

 are very remarkable. They are most remarkable in certain forms 

 of sea-cucumbers, or holothurians, belonging to the Echinodermata,, 

 plump animals, whose bodies are covered by a tough, brown, 

 leather-like skin and resemble a cucumber. If Holothuria Poll, e.g.,. 



1 Cf. p. 108. 



