We have first an embryo of twenty hours of incubation; following it one of one day. You can observe just a little line of structure indicated and showing where the longitudinal axis is to be situated. By the second day the chick has distinctly a head and a little heart, and those who are expert can differentiate with a microscope the axis of the body, the beginning of the formation of the intestine and of the muscles. At the end of the first day there was little more than a mere gathering of cells, but during the twenty-four hours of the second day the gathering has changed from a mere streak upon the surface of the yolk to a well-formed individual, with recognizable parts and several times the volume it had when one day old. The next figure illustrates the alteration which occurs during, approximately, the third day. It is obvious that the embryo has again made an enormous increase in volume. The eye has developed, the heart has become large, the tail is projecting, the dorsal curve of the future neck is distinguishable. We pass next to the fourth day. Is it not a strange looking beast, with its wing here and leg there, a little tail at this point; an enormous eye, almost monstrous in proportion; and, finally, here a bit of the brain. After five days we have a chick the brain of which is swelling, causing the head to be of so queer a shape that, with the eye, which seems out of all proportion to the rest of the body, it imparts an uncanny look to the embryo. The wing is shaping itself somewhat, and the ends of the leg, we can see, will, by expansion, form a foot. Finally, the chick after seven and after eight days is figured. In the short interval of only six days the chick grows from the size represented by Fig. 2 to that shown in the last figure upon the plate. It is an enormous increase. Suppose a chick after it was born were to grow at such a rate as that! The eight-day embryo is thirty or forty times as big as it was eight days before. It would seem marvelous to us if a chick after it was hatched should become in eight days thirty times as large and heavy as when it first came out from the egg. It is perhaps advisable to let you follow the growth of the chick a little farther, and accordingly I present another picture which shows an embryo of about ten days. The little marks upon the surface of these embryos indicate the commencing formation of the feathers. A comparison of the series of figures proves that the development is taking place with marvelous speed. We need only to look at these stages, comparing them with one another, to realize that the progress of the embryo in size and development occurs with a rapidity which is never to be found in later stages.
The history of embryonic rabbits declares with equal emphasis that the earliest development is extremely rapid. I wish now to show you
The final slide of the series showed a chick of three and one half days. It has not seemed necessary to reproduce these figures with the present text, as they merely duplicate, on a larger scale and with more detail, the pictures which have been included.
