Page:Popular Science Monthly Volume 71.djvu/117

of calcareous material. Nothing like cartilage, nothing like bone, exists in the early state of the embryo. They represent something different and new. The next of our illustrations shows us a muscle fiber of the sort which serves for our voluntary motions, which is connected typically with some part of the skeleton. These muscle fibers are elongated structures. Each fiber contains a contractile substance different from protoplasm, and which exists in the form of delicate fibrils which Fig 15. Part of a Muscle Fiber of the Human Tongue to show the Cross Striations. Two nuclei are included, one of which is shown at the edge of the fiber, the other in surface view. In the adult striated muscle fibers of mammals the nuclei are superficially placed. run lengthwise in the muscle fibers, and is so disposed, further, that a series of fine lines are produced across the fiber itself, each line corresponding with a special sort of material different from the original protoplasm. These cross lines give to the voluntary muscle fibers a very characteristic appearance, in consequence of which they are commonly designated in scientific treatises by the term striated. A striated muscle fiber is that which is under the control of our will. It should perhaps be mentioned that the muscle fibers of the heart are also striated, though they differ very much in other respects from the true voluntary muscles. And last of all for this series of demonstrations, I have chosen a representation of the retina. One can see at the top of the figure the peculiar cylindrical and developing projections, which are characteristic of a retina, projections which are of especial interest because they represent the apparatus by which the rays of light are transformed into an actual sensory perception. After this has been accomplished, the perception is transmitted into the interior substance of the retina, and by the complication of the figure you may judge a little of the complication of the arrangements by which the transmission through this sensory organ is achieved, until the perception is given off to a nerve fiber and carried to the brain. There is not time to analyze all I might present to you of our present knowledge concerning the structure of the retina. But it will, I think, suffice for purposes of illustration to call your attention to the complicated appearance of the section as a whole and to assure you that nothing of the sort exists in the early stage of the embryo. To recapitulate, then, what we have learned from the consideration of these pictures, we may say that in place of uniformity we now have diversity. It should be added, to make the story complete, that the establishment of this diversity has been gradually brought about, and that that which