400 OF SECRETION. 



tion of new cells does not take place from a single germinal spot at the 

 extremity of the follicle, but from a number of points scattered through 

 its entire length. In the second type of Glandular structures, the 

 parent-cell does not remain as a- permanent follicle ; but, having come 

 to maturity and formed a connexion with the excretory duct, it dis- 

 charges its entire contents into the latter, and then shrivels up and 

 disappears, to be replaced by newly-developed follicles. In each parent- 

 cell of a gland formed upon this type, we shall find all its secondary or 

 secreting cells at nearly the same grade of development ; but the differ- 

 ent parent-cells, of which the parenchyma of the gland is composed, are 

 in very different stages of growth at any one period : some having dis- 

 charged their contents and being in progress of disappearance, whilst 

 others are just arriving at maturity and connecting themselves with the 

 excretory duct ; others exhibiting an earlier degree of development of 

 the secondary cells ; others presenting the latter in their incipient con- 

 dition ; whilst others are themselves just starting into existence, and as 

 yet exhibit no traces of a second generation. The former seems to be 

 the usual type of the ordinary glands ; the latter is chiefly, if not en- 

 tirely, to be met with in the Spermatic glands. 



2. Of the Liver and ike Bile. 



720. The Liver is more rarely absent than any other Gland ; being 

 discoverable, under some form or other, in all but the very lowest mem- 

 bers of the Animal kingdom. Its simple condition in the higher Po- 

 lypes has been already noticed ( 716) ; and it is met with, under an 

 almost equally simple form, in the Starfish. As we ascend the scale, 

 however, we find it assuming a much greater importance, and presenting 

 a great increase in size. This is particularly the case in the Mollus- 

 cous classes ; and also in the Crustacea, a class which, in mode of respi- 

 ration and in general habits, bears a great resemblance to the Mollusca. 

 In nearly all such animals, the Liver makes up a large proportion of 

 the mass of the body. It usually consists of a series of large follicles, 



Fig. 116. Fig. 117. 



Lobule of Liver of Squilla Mantis; exterior. Lobule of Liver of Squilla Mantis, cut open. 



which branch out into smaller ones (Figs. 116 and 117), and of which 

 several open into one excretory duct ; but these ducts remain separate, 

 and discharge their contents into the intestine by several distinct ori- 



