4th, Cells larger than the last, assuming more of a globular form, still closed, full of nu constant state of change, passing through stages of development, maturity, and atrophy.
" 2d. The state of change is contempo raneous with, and proportional to, the forma tion of the secretion, being rapid when the latter is profuse, and vice versii.
" 3d. The acinus is at first a single nu cleated cell. From the nucleus of this cell others are produced. The parent-cell, how ever, does not dissolve away, but remains as a covering to the whole mass, and is ap pended to the extremity of the duct. Its cavity, therefore, as a consequence of its mode of development, has no communication with the duct. The original parent-cell begins to dissolve away, or to burst into the duct, at a period when its contents have attained their full maturity-. This period varies in different glands, according to a law or laws peculiar to each of them.
" 4. The secretion of a gland is not the product of the parent-cell of the acinus, but of its included mass of cells."* An ideal representation of these successive changes is given infig. 322. At b is seen a cleated cells, and situated mare towards the surface of the lobe (e). 5th, The full sized vesicles situated at the surface of the lobe, with their contents in various sta,ges of deve lopment (d, e, f). These vesicles are spheri cal and perfectly closed ; that part of the wall of each, which is attached to the hollow pedicle, forming a diaphragm across the pas sage, so that the vesicle has no communica tion with the ducts of the gland. The con tained cells are at first spherical (g) ; but as the spermatozoa are gradually formed within them, they present a cylindrical form (h), and they are arranged within the vesicles in somewhat of a spiral manner (f). When the development has advanced to this stage, the diaphragms across the necks of the vesicles dissolve away or burst ; and the bundles of spermatozoa float along the ducts of the gland, some of them separating into indivi dual filaments (i , k) . Besides the bodies now described, ProE Goodsir has observed what he considers to be vesicles which have discharged their contents, and which are in a state of atrophy (fig. 322. a).
The testis of Squalus cornubicus, the lime tional history of which has been now given, is considered by Prof. Goodsir as a type of a number of glands, whose action takes place after the same manner ; and he lays down the following general facts, which he has ascer tained in regard to glands of this order.
" 1st, The glandular parenchyma is in a portion of gland with two acini ; one of them being a simple primary cell, the other being in a state of development, its nucleus pro ducing young cells. At c, both acini are ad vancing ; the second has almost reached ma turity. At d the second acinus is ready to pour out its contents, and the first to take its place ; and at e, the second acinus is thrown in a state of atrophy, whilst the first is be come fully matured.
There is another set of glands, in which the follicles remain persistent for a much longer period, and continue to produce many suc cessive generations of secreting cells ; of these, the liver of the Crustacea may be taken as the type. From the appearances presented by these follicles, which have been already figured and detailed (fig. 312.), it seems fair to conclude, that the development of these cells takes place from the cmcal extremities ; and Prof. Goodsir considers that they originate in a " germinal spot," which is the persistent nucleus of the parent cell, whose enlargement and connection with the gland forms the follicle. Growth in glands of this kind is regulated, according to him, by the following laws : — " 1st. Each follicle is virtually permanent, but actually in a constant state of develop ment and growth.
" 2d. This growth is contemporaneous with the function of the gland ; that function being merely a part of the growth, and a conse quence of the circumstances under which it occurs.
" 3d. The vital action of some follicles is continuous, the germinal spot in each never ceasing to develope nucleated cells, which take on the action of, and become, primary secreting cells, as they advance along the follicle. The action of other follicles is periodical.