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Engystoma Texense, Girard — Part 5
Development begins within two or three hours after the eggs are laid. To the naked eye the change seems slight for the first two days. It begins by the egg becoming slightly larger. The black spreads until the visible white portion becomes no larger than a pin-point. However, if we increase the power of our eyes ever so little by the use of a magnifying-glass of any kind, we can see something more of the marvellous changes that have begun and will continue from the vital force of the life within until the egg has become a frog in miniature. If we examine the eggs with a lens very soon after they are laid, we can see that a groove appears in the midline at the top of each. It increases in length until it encircles the egg. The partition of which this is the external evidence divides the egg into two equal parts. Many scientists believe that these correspond to what will be the right and the left side of the young frog. This groove is visible to the naked eye, though somewhat difficult to see. A second groove appears at right angles to the first and rapidly encircles the egg until the latter is divided into four nearly equal parts. The next division is made by a horizontal partition instead of a vertical. This partition and its external groove are somewhat above the centre of the egg, so that the upper portion is smaller than the lower. From this time on the division is more rapid and more irregular. It is possible to follow it somewhat farther with just a hand-lens, but soon the parts (cells) become too small and too many to be distinguished.
If we look carefully at the eggs on the afternoon of the second day after they are laid, we shall see a dark line of crescent shape on one side, just below the middle (equator).Twelve hours later the crescent has become a circle. The black surface of the egg seems to have spread quite to the edges of this circle. The circle encloses a mass of white yolk, which protrudes like a small cushion. (For technicalities concerning the formation of the blastopore and its subsequent history, see Bibliography, pp. 241-250.) Throughout the development, our knowledge will be limited to what we can see on the outside. We can know very little, or nothing, about the rapid and wonderful happenings within. However, certain external changes will indicate internal conditions. This circle with its protruding plug of yolk is the external sign of a splitting or separation between cells, which is the beginning of the formation of the digestive tract. This digestive tract begins forming at its posterior instead of at its head end, therefore this yolk-plug marks a point very near the posterior end of the young frog.
Before the third day is over, the eggs have become elongated and on many of them a visible groove is being formed lengthwise along the top. This groove marks the back of the developing frog; so that now we know which is to be the head and which the tail end, and which is the right and which is the left side.
By the fourth day, the eggs are greatly changed. The egg has become still more elongated and the yolk-plug has wholly retreated. The groove which began so simply has extended along the whole length of the back (top of egg) and two great folds of the surface are slowly rolling in over it, one on each side. These folds are broader at the head end. This is perhaps the most interesting part of the development that we shall see. This groove and the folds which are rolling over it will form a tube within, which is the beginning of the nervous system. The head end of the tube forms the brain and the remainder makes the spinal cord.
On the fifth day, so great has been the advancement that we are again puzzled to account for appearances. The elongation is still greater, and head and tail ends are curved somewhat to one side. The line of the back is nearly straight, the yolk side (front) is very convex. The nerve-tube seems to be wholly closed in, but the folds still persist to a certain extent over the head and along the back. The projection which is to form the tail is very apparent.
If we now look at the embryo from above or below, the region of head and neck shows slight projections. Examining these very carefully from the side, we see the appearance shown. That portion in the most anterior and largest projection or swelling (sense plate), will become the facial part of the head, with upper and lower jaws, mouth, and nostrils. The three smaller elevated portions back of this are destined to make the gills. There is a blunt projection on the lower portion of the head end, which, on account of its position (the front margin of the sense plate), might be mistaken for a mouth. Viewed from the front it becomes one side of a horseshoe-shaped structure which will make, not the mouth, but a pair of "sucker," to be used by the tadpole to attach itself to water-weeds.