454 



SECRETION. 



[CHAP. xxxn. 



directly continuous with that of the outer surface, on which the 

 duct opens. This, for example, is the case in the sweat glands, in 

 the sebaceous glands, already described (pp. 422, 423, vol. i.), and 

 in the kidney. But in other instances, whether owing to the form 

 assumed by the terminal extremities of the duct, or, to an actual 

 difference in the arrangement of the glandular epithelium, the 

 greatest difficulty exists in determining the exact nature of the 

 anatomical relation of the glandular and ductal epithelium, and of 

 ascertaining whether they are continuous at all times or only at 

 certain periods when the secretion is discharged. For example, in 

 the salivary glands, the pancreas, and some others, the terminal 

 parts of the gland are vesicular, while the ducts are tubular. 

 Vesicular terminations of the ducts may be rendered apparent by 

 mercurial injection ; but it is by no means easy to demonstrate 

 the permanent continuity of the epithelium lining or filling these 

 vesicles with that which lines the ducts. 



It is probable that most or all gland-follicles were originally 

 closed vesicles. This has been shown by Dr. Allen Thompson to 

 be the case in the early condition of the gastric glands; and 

 Henle applies this view to other gland-structures. In his beautiful 

 investigations upon the testicle of Squalus Cornubicus, Professor 

 Goodsir has shown that the primary or mother cells are deve- 

 loped by the growth of isolated nucleated cells attached to the 

 side of the duct. These cells grow and become the future acini, 

 being connected with this duct by a hollow pedicle; in their 

 interior, secondary cells, in which the spermatozoa are developed, 

 appear. Still the mother cell is a closed cavity, and separated from 

 the duct by its original wall. At length it gives way at this point 

 and discharges its progeny of cells, containing spermatozoa, into 

 the duct. The follicles then appear to become atrophied. There is 

 another class of gland-structures, in which, according to this view, 

 the mother cell remains persistent after it has discharged its first 

 progeny of cells, and continues to produce successive generations of 

 cells ; the development of these cells commencing at the csecal ex- 

 tremity of the follicle. The point at which they originate is looked 

 upon by Professor Goodsir as the persistent nucleus of the original 

 parent cell; and it is termed by him, the "germinal centre." 

 Although it is scarcely possible to demonstrate the existence of this 

 germinal centre in the gland-follicle, there are many instances in 

 which this view certainly appears to be borne out by the structure 

 of the part when subjected to a careful microscopical examination. 



