LUBRICATING TISSUES 395 



much thickened inside, that it practically fills it up and leaves very little 

 lumen to be seen. This gives the gland a solid appearance with the basal 

 cells of the stratified epithelium at the fundus. These basal cells appear 

 to be in the same condition that they exist in other parts, but the distal 

 layers become larger as they approach the surface of the lumen or, if 

 there is no lumen, the neck of the gland. Knowing that stratified epi- 

 thelium proliferates from the basal layer, we can follow the history of 

 these cells of the sebaceous glands and see that, after a cell has been 

 divided off from the basal layer and started on its journey towards 

 the surface, it undergoes changes that are very different from the well- 

 known changes seen in other and unspecialized parts of the outer epi- 

 thelium of man. 



Instead of forming keratin it goes through a process of vacuolization 

 in which the vacuoles are filled with the oil which has been elaborated 

 at the expense of, and through the destruction of, the cytoplasm. As 

 the elaboration of oil continues, the nucleus disintegrates so that the 

 production of oil in these glands results in the destruction of the 

 cell. 



The oil drops begin to appear in the part of the cytoplasm next the 

 nucleus, as numerous and small vacuoles. They rapidly increase in size 

 until they fill the cytoplasm, and by the time that the cell has reached 

 the surface of the epithelium, or neck of the gland, it appears as a mass 

 of oil drops inclosed in a sac, the cell-membrane, and containing the 

 remains of the nucleus. A number of these ripe cells collect at the fundus 

 and rupture to form the glandular discharge. The secretion thus contains 

 the degenerated nuclei and cell-membranes of the cells that produced it 

 and which, we can now see, were completely sacrificed in the production 

 of one portion of the secretion of the gland. This is not the case with 

 most glands. 



Sebaceous or Lubricating Glands in the Birds; Oil Glands of the 

 Chicken. Here are found two large groups of sebaceous glands lying 

 parallel with one another (or nearly so), and all the glands of each group 

 opening into a single cavity, itself an invagination of the integument 

 on the rump of the animal. Each of these glands is long and tubular 

 in shape, with a wide and clearly defined lumen which is of equal width 

 for its entire length. The gland shows no differentiation into the neck 

 and fundus. Its long sides are everywhere lined with an epithelium 

 that is very evidently a- stratified epithelium of some ten or twelve 

 irregular layers (Fig. 359). These layers all come from a basal layer 

 that proliferates them exactly as in an ordinary stratified epithelium, 

 with the important exception that no amitotic divisions take place in 

 the cells that are moving to the outer surface. 



The first two rows of these cells show no change except, perhaps, an 



