Page:Popular Science Monthly Volume 71.djvu/374

368 cells, and the cells had some protoplasm, but, gradually, as development progressed, there appeared in them longitudinal fibrils different from the protoplasm, and the fibrils also created ultimately the appearance of cross lines on the fiber. It is the fibrils which perform the muscular contractions. It is not the original unmodified protoplasm, but the modified or differentiated muscular cell which is capable of voluntary contraction.

The next picture (Fig. 47) shows us clearly and strikingly how much the differentiation may vary. We have here another type of differentiation. These are gland cells; we can see here, as I pointed out to you before, the material in the form of granules, which is to produce the secretion from these gland cells. This is an orbital gland, and here are the cells, which are very much smaller because they have discharged their secretion. Three of the cells are represented separately. The first shows us a cell full of the material which is to be discharged and is to form a part of the secretion of the saliva. The second is a cell which has partly lost its accumulated material, and the third is one which has discharged it almost completely, so that it has become very much reduced in size. We learn from such structures as these that the size of cells may vary also according to their functional condition. We have here a similar gland. This is sometimes called the salivary gland of the intestine, better termed the pancreas. Here we can see for each of these cells a nucleus and a body divided into two parts, a darker portion around the nucleus and a lighter part with little granules in it, which represents the accumulation of material which is to form the secretion. When the cells have discharged their secretion, they, like the cells in the salivary gland, are found to have diminished in size and become very much smaller indeed than they were in their earlier state when charged with the zymogen destined to be given out. In this case also we have an illustration of a functional variation in the size of the cells. This ends the series of pictures which I wanted especially to show to you as illustrating the changes of the cells as their differentiation progresses. We can see in the bodies of the cells the changes which have occurred.

Here is a picture which teaches us one thing more about these cells. Notice the scattered nuclei, each surrounded by protoplasm, completing the cell. The protoplasm of each of these cells is connected across with the protoplasm coming from another, so that the whole set of cells forms an irregular photoplasmic network. Now in the spaces between these cells are fine lines. These represent delicate structures which we call connective tissue fibrils, which have a mechanical function. By their tensile strength, their power to resist and pull, they give a certain supporting power to the tissues. Our picture represents one of the tissues which support and connect other portions of the body. Now the fibrils apparently lie entirely disconnected from