1. Glands - a general structure:

The System level - like an underlying "syntax" - becomes formulated in differentiation of glands on all following organ, tissue and cell levels.
   Glands are formed from all kinds of epithelium. Glands means actually differentiation as such between substances produced by epithelial cells.

The basic model for glands seems given already on the gastrula stage of embryo development: an already specialized surface layer of cells, which faces on to outer room and an inner cavity and invaginates from the surface in certain areas in similarity with how gland tissue is descended epithelium.
   The whole archenteron could in this sense be regarded as a primary gland, as well as the neural tube with its secretion of cerebrospinal fluid (CSF).

2. Two special glands:

The nervous system and nutrition system as two opposite directed vector fields meet during the evolution in two glands on the organ level: in hypophysis and adrenal glands composed of tissue from both. They become complex combinations of those poles (or partial structures) from underlying level of higher dimension degree (d-degree) on a level of lower d-degree.
   (Systems →> Organs as a d-degree step 4 →>3 in the level chain in our interpretations here.)
   The information via hormones is chemical as are the transmitters in the very synapses of the nervous system. The way of information goes the way of the nutrition system via the blood system outwards.

      

Fig Gl-1-37-1

In this meeting of poles in hypophysis and adrenal glands there is also a geometrical "pole exchange" in terms of the dimension model:
   Nervous tissue (00-pole in relation to nutrition system from 0-pole) becomes the inner, the center, the marrow of adrenal glands from cells in the sympathetic nervous system, and the tissue from the nutrition system (coelom, indirectly from vegetative pole) becomes anticenter, the adrenal cortex.
      Fig Gl-2-37-2

In hypophysis it's tissue that has immigrated from the throat (pharynx) that forms a pocket around the nervous tissue.
   (It's certainly said that the pocket derives from ectoderm, not the vegetative pole in the embryonic development. Yet, throat is part of the digestive canal in the nutrition system.)

The hypophysis is historically a later development in evolution and it may be asked which more such combined organs that could follow with a further biological development? Is there actually any other reason than a dimensionally given "pole meeting" that biologically - chemically gives cause for adrenaline cortex and adrenaline marrow (medulla) to combine to one organ, when they produce different types of substances (cortex: steroids as cortisol; marrow: adrenaline and noradrenaline)?

Pancreas with its mixed gland tissues seems to represent a secondary type of "pole meeting" within the nutrition system, In humans it develops as a compound of one ventral and one distal "bud" or invagination from the intestine, hence from a polarity of second order. It is also two-way directed in its function with both endocrine and exocrine secretion.

3. Gland shapes:

As glands are epithelium invaginating inwards, the macrostructure of glands seems dimensionally develop inwards also in the sense of towards higher d-degrees in shape: from single cells to "linear" tube forms to branched, hollow tubes along 2-3 coordinate axes - to branched, cell-filled tubes - to centered, cell-filled organs with only canals connecting to the surface of epithelium. These latter develop often radially ordered cell lines as of pole 3b in our model and get own blood supply.
   Hence, it's a development as in d-degree steps 0 →> 1 →>2 →> 3. The liver for instance has first only a tube form.

Fig Gl-3-38
(Cf. figure b with how coelom first grow out from archenteron in the embryo.)

Surface epithelium is often of squamus type (low, flat), while epithelium of glands often is cubic (cuboidal): Such a detail points to a d-degree step of type 3←2 already on the level of single cells. (Stratified epithelium may have up to 6 layers. Cf. 6 layers in the cerebral cortex.)

4. Differentiation of glands:

We could see the differentiation in terms of polarities related to the dimension model: in directions, in center - anticenter formation (as mass - space) and in positions along coordinate axes - naturally besides type of secreted substances:
   On the level of the whole organism there is the polarity between glands with secretion outwards the surroundings and those with secretion inwards: skin glands versus internal ones.
   On the organ and tissue levels there is the polarity exocrine - endocrine glands, directions of secretion (poles 4b - 4a).
   The general development seems to go from exocrine secretion outward cavities to more endocrine secretion, inward blood vessels.
In centered organs the polarity center - anticenter as marrow - cortex appears as the d-degree relation 3 - 2.

The differentiation along the Front - Back axis of the digestive canal in relative positions is reasonably to a certain degree related to kind of passing substances, carbohydrates, proteins, fatty acids, but that's surely only a part of the truth, underlying polarities of the coordinate axes another.
   A sketch:
      Fig Gl-4-33-3

Generalized it seems possible to regard the whole body more or less as a hierarchy of glands. Simultaneously the development of glands implies a continuous growth of inner surfaces from the first embryological stages (like the growth of a coastline towards infinity when one goes deeper and deeper into details). A question is if position on a surface is a necessary condition for the differentiation of cells in genetic activity - or the reverse, a result of it?

To 05. Blood system