1. Every gland is formed from a portion of the primary plastic and amorphous mass ( blastoderma) of which the body of the embryo consists.
2. This mass is at first gelatinous, extremely delicate and diaphanous ; it subsequently be comes thicker and less transparent. In the beginning it is solid, and in the case of those glands which are appended to the alimentary canal,—that is to say, the salivary glands, the liver, and the pancreas, (and the same laws are observed in the formation of the lungs,*) it appears as a projection on the mucous mem brane. ( Fig. 222, A.) 3. In a short time this rounded mass begins to project on its external surface, and thus forms a number of lobes, or, as it were, little islands, which, by the continuation of the same process, become more and more numerous and smaller in size; and thus,according to the cha racter of the gland examined, are at length formed all the minute lobes of which it con sists. ( Fig. 222, B.) 4. Simultaneously with this development of the outer surface of the plastic mass, but quite independently of it, a metamorphosis is going on within, by which the internal canals, which subsequently become the secreting tubes, are formed. In the first instance a hollow or cavity is noticed communicating with the tube of the intestine, and which subsequently be comes the principal or excretory duct. When it first appears it is a simple sac, (fig. 222, C,) but in proportion as the projections or lobes are formed on the external surface, lateral branches are added to the principal duct ; and these again become more and more ramified, till an indefinite number of tubes are formed. ( Fig. 222, D.) nishing the aorta (b) dividing into its right and left trunks, together with the principal venous trunk (c), are represented ; d is the intestine, f the rudiment of the corpora Wolffiana, and g g the rudiments of the upper and lower extremities.
One of the most remarkable differences ob served in the development of the several glands relates to the proportion between the mass of the primary plastic substance, and the extent and number of the contained tubes ; thus, in the evolution of the liver there is seen a thick layer of the primitive matter ; whilst, on the contrary, the parotid gland in the embryo of a calf two inches seven lines long, consists of a tube visible to the naked eye, and not at all covered by parenchyma.
5. The mode in which the secondary tubes are developed has been observed with great care; and it is distinctly established that they du not proceed as mere elongations of the pri mary cavity, but are formed in an indepen dent manner. One of the latest writers on the
development of the body, Valentin • has given a very exact account of the process in all the glands. He states that in the neighbour hood of the chief duct or of a branch of it, small oblong accumulations of the plastic mass are fonned, which become hollowed in the interiur, and these hollows, at first inde pendent the principal cavity, subsequently communicate with it. It is also observed by Muller that in the kidney of I3atrachian Am phibia, the secreting tubes first appear as vesicles which are formed before the ureter, and therefore independently of the principal duct.* As the tubes become more developed, the plastic substance around them, by acquiring greater firmness, constitutes their walls, and thus determines their exact form and limits. It is necessary to state that in every instance without an exception, the newly-formed canals end in coical extremities, which nre often rather swollen, presenting a pedunculated ap pearance.
6. In proportion as the canals become formed in the substance of the plastic mass, this latter gradually diminishes in quantity, till ultimately, when all the tubuli are formed, it is so much reduced that it merely fills up the interlobular fissures, and is in fact converted into the interstitial cellular tissue.
7. At the same period of time that the tubes are thus being formed, the bloodvessels are being developed ; and, as Muller and Valentin remark, a very close parallel is presented in the generation of these the essential parts of the gland. As in the case of the tubes, there are at first little masses, or islands, of the plastic substance, which suhsec3uently join together, and their interior becoming liquified, a num ber of little channels are formed containing a circulating fluid, and which channels, by the subsequent consolidation of their walls, are at length formed into perfect bloodvessels. Like the tubes these vessels are at first independent; they afterwards open into larger trunks and ultimately into the heart. It is proper to remark that, although there is such a corres pondence in the process of development in each instance, the bloodvessels are formed quite independently of the canals ; that they occupy a different part of the plastic mass; and that they never present that continuity which ought at this epoch to have been very apparent, if the theory of Ruysch had been founded in truth.