390 THE SOLIDS. 



with the internal and external integuments of the body, 

 the skin, and mucous membrane, since in very many glands, 

 the secreting structure is contained within, but most probably 

 not developed from inverted processes of these tissues. Such 

 an arrangement of the glands* although a very natural one as 

 far as it goes, would yet not embrace all the glands ; the 

 vascular and some others would be excluded from it. 



The proof of the position that very many glands, and even 

 the most complex ones, are contained in offsets of the various 

 modifications of mucous membrane and outer integument, 

 observers have stated to be derived from an examination of 

 these organs in an embryonic condition, when even, it has 

 been affirmed, the most elaborate and varied of them may 

 be seen as simple follicles or sacks springing from the general 

 membrane covering the skin or lining the internal cavities. 



It would appear, however, from recent and trustworthy 

 observations, that the principal glandular organs of the body 

 have each a separate development, and that it is only after 

 they have been more or less completely formed that they 

 become attached to the surface of the skin or mucous mem- 

 brane, upon which their secretions are poured. 



Numerous divisions of glands have been proposed by dif- 

 ferent writers : the majority of these do not require any 

 special notice : there is one, however, originating with Mr. 

 Goodsir, which would appear to be ingenious and philo- 

 sophical, which deserves especial mention. That gentleman 

 divides glands into two types or classes ; the distinctions 

 existing between which are founded upon observations de- 

 rived from the study of the early development of these organs. 



In the first type of glandular structure, the follicles which 

 are at first distinct from the excretory duct, but which sub- 

 sequently become united with it, are regarded as parent cells, 

 the granular cells contained within them being secondary 

 formations, which are continually in process of development, 

 springing from a germinal spot situated either at the bottom 

 of the follicle, if this be short, or over its entire surface, if the 

 follicle be tubular : in this class the follicle is a permanent 

 structure. 



