There are various modes of origin of the lymphatic vessels which are more or less characteristic of the different parts in which they occur.
Fig. 87. Endothelium from serous surface without stomata (nitrate of silver).
In the connective and allied tissues there are variously-formed fissures or clefts, which can be filled with fluid forced into the tissues by puncturing the skin with the nozzle of a fine syringe, such as is used for hypodermic injection.
These fissures contain the protoplasmic units of the tissue, and transmit the ordinary transudation stream for nourishing the tissues. They freely communicate with one another, and lead into the beginnings of the network of lymphatic capillaries.
The lymph capillaries run midway between the blood capillaries, and are made up of a single layer of nucleated endothelial cells, which can be brought to light with silver staining.
In some tissues, such as that of the central nervous system, liver and bone, the lymph vessels commence as channels encircling the blood vessels, or perivascular lymph spaces. Here the lymph channels form a kind of sheath for the minute blood vessels, and pass along to the connective tissue of the adventitia of the larger.
The lymph vessels may also be said to commence on the surface of serous membranes which are intimately connected with the lymphatic system, and may indeed be regarded as nothing more than inordinately developed lymph spaces. In most parts of the endothelial surface of serous cavities are a number of so-called stomata, or small apertures surrounded by a few cells, which differ from the ordinary endothelial cells in many respects, and probably have to control the passage of the fluid from the serous cavity into the lymph vessels. These stomata are found to be placed at the commencement of the dense network of lymph capillaries, which lies in the subserous tissue.
Fig. 88. Endothelium from serous surface with stomata surrounded with granular protoplasmic cells.