i INTERNAL PEOTECTIVE SECRETIONS 7 



by areolar connective tissue, and richly provided with blood and 

 lymph vessels. The size of the vesicles varies considerably ; the 

 largest may be one millimetre in diameter, so that they are 

 visible to the naked eye. They are rounded or oval in form, 

 with a wall consisting of a single layer of cubical or columnar 



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epithelial cells, which are the secreting elements (Fig. 2). 

 According to Langendorff two kinds of cells can be dis- 

 tinguished : " Hauptzellen," which have sharp outlines and shin- 

 ing, finely granulated protoplasm ; " Colloidzellen," which have 

 indefinite outlines and protoplasm filled with large granules, 

 shown by their affinity for certain pigments to consist of colloidal 

 substance. The first are young cells that secrete by exudation ; 



p [o . _>. Thyroid gland of infant. Vesicles of various sizes, lined with single layer of cubical 



epithelial cells. 



the second are older and exhibit a marked secretory activity, 

 during which they liquefy and break up, so that both protoplasm 

 and nucleus pass into the secretion. In fact, the colloid 

 fluid of the alveoli contains both the old disintegrated epithelial 

 cells, and leucocytes that have emigrated from the blood 

 capillaries, as well as erythrocytes in process of destruction and 

 discoloration. 



Liibcke (1902) concluded from histological observations, more 

 particularly of fresh preparations of the gland, that the so-called 

 "colloid" cells are only artificial products, due to the diffusion of the 

 contents of the vesicle in and around the atrophied epithelial cells. 

 In any case they would not represent the secreting cells. Accord- 

 ing to this author the thyroid vesicles contain a homogeneous fluid, 

 which is not shiny, and is quite distinct from the protoplasm, 

 being watery or gelatinous in consistency. It can be washed out 



