DIFFERENT FORMS OF GLANDULAR STRUCTURE. 399 



the history of the development of Glands in general, it appears that the 

 follicles may be considered as parent-cells and that the secreting cells 

 the interior constitute a second generation, developed from the 

 mclei germinal spots on the walls of the first. Of such parent-cells, we 

 tve characteristic examples in the Peyerian glandules of the intestinal 

 mal ( 450), and also in the Thyroid gland ( 513) and Supra-Renal 

 capsules ( 510) ; and the most elaborate glands, in their earliest stage 

 )f development, present a similar condition. These closed cells become 

 follicles, by opening at one extremity, either upon a neighbouring free 

 face, or into a canal which is prolonged from it. Thus the first rudi- 

 lent of the Liver is formed by a thickening of the cellular mass in the 

 rails of the alimentary canal, at the spot in which the hepatic duct 

 subsequently to discharge itself. This thickening increases, so as 

 form a projection upon the exterior of the canal ; and soon after- 

 Is the lining membrane dips down into it, so that a kind of caecum 

 formed, surrounded by a mass of cells, as shown in Fig. 108. The 

 ncrease of the mass seems to take place by a continual new budding- 

 >rth of cells from its peripheral portion, which takes place to a consi- 

 lerable extent before the caecum in the interior begins to ramify. Gra- 

 dually, however, the cells of the exterior become metamorphosed into 

 fibrous tissue for the investment of the organ ; those of the interior 

 break down into ducts which form continuations of the principal canal ; 

 whilst those which occupy the intervening space, and which form the 

 bulk of the gland, seem to be developed into follicles, and to give origin 

 the proper secreting cells. As this is going on, the hepatic mass is 

 -adually removed to a distance from the wall of the alimentary canal ; 

 ind the caecum is narrowed and lengthened, so as to become a mere con- 

 necting pedicle, forming, in fact, the main trunk of the hepatic duct. 

 development of the Pancreas, Salivary glands, &c., seems to follow 

 the same plan. 



719. It has been pointed out by Prof. Goodsir, that the continued 

 development and decay of the glandular structure, in other words, the 

 elaboration of its secretion, may take place in two different modes. In 

 one class of Glands, the parent-cell, having begun to develope new cells 

 in its interior, gives way at one point, and bursts into the excretory 

 duct, so as to become an open follicle, instead of a closed cell : its con- 

 tained or secondary cells, in the progress of their own growth, draw 

 into themselves the matter to be eliminated from'the blood, and, having 

 attained their full term of life, burst or liquefy, so as to discharge their 

 contents into the cavity of the follicle, whence they pass by its open 

 orifice into the excretory duct : and a continual new production of secon- 

 ary cells takes place from the germinal spot, or nucleus, at the extre- 

 lity of the follicle, which is here a permanent structure. In this form 

 )f gland, we may frequently observe the secreting cells existing in vari- 

 ms stages of development within a single follicle ; their size increasing, 

 ind the character of their contents becoming more distinct in propor- 

 tion to their distance from the germinal spot (which is at the blind 

 irmination of the follicle), and their consequent proximity to the outlet 

 (Fig. 114). In some varieties of such glands, however, as in the 

 greatly-prolonged follicles or tubuli uriniferi of the kidney, the produc- 



