8. The several glands are not developed equally early, some having their organization much more advanced than others; thus at the time when the pancreas is so far formed as to contain an immense number of canals, the parotid presents only a single duct or a few ramifications.f The principle which regulates the relative degree of development has evi dently reference to the importance of the organ during the foetal life ; and in this respect the liver is most for that body being, as I conceive, the true decarbonising organ in the animal kingdom, and therefore its func tions being doubtless necessary in the foetus, very quickly acquires a high degree of organi zation, so much so that, as we learn from all observations, it very speedily fills the greatest of the abdomen.:This development of circal tubuli is seen in the liver of Limns as stagnalis in the em bryo state, (fig. 223). In the embryo of Lacerta vi ridis, (fig. 224,) the rudimen tary liver with its blind se creting canals (e) are observed; the elongated heart (a) fur Lastly, The laws in obedience to which the glands are developed, are as universal as to their existence in the animal kingdom, as those which regulate the formation of the nervous, osseous, and vascular systems ; and thus it may be noticed that the complex glands of man and the mammalia, such as the parotid, the pancreas, and the liver, pass, in the various epochs of their development, through those forms, which in the lower animals, and espe cially in the invertebrated tribes, constitute the permanent structure. It may also be stated,
that when any particular gland first appears in the animal series, it presents the most simple structure, although the same gland in the higher classes acquires the highest degree of complexity. Thus, the liver in insects is tu bular, and in many of the amphibia excavated into large cells; the pancreas in fishes consists of separate tubuli; the salivary glands of birds are extremely simple ; so also are the mam mary glands in the Cetacea, and the prostate glands in many of the Mammalia.