Growth and Education

Book Description:

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER III HINTS FROM EMBKYOLOGY In this chapter we can notice but a very few of the most important facts of embryology, and these only in so far as they throw light on growth and education. If you look at an egg dropped on toast or poached, you will notice a lighter spot on the yolk, about as big as the end of a small lead-pencil, which looks as if some one had blistered it with a hot iron. It is a disk like a watch-crystal, composed of a multitude of cells. Development in a fresh egg has already gone far beyond the stage of the single cell, of which every egg consists at first. This is an embryo chicken, without a single organ, and with hardly a tissue except the two distinct layers of which it consists. Soon the embryo becomes elliptical or shield-shaped, and we find a rod running lengthwise through its middle line. This is the notochord, the beginning of the vertebrate skeleton. Just above and parallel to the notochord a tube appears, the beginning of brain and spinal marrow. We can gain a fair idea of the position of these two organs, if we thrust two hatpins through a griddle-cake parallel to its diameter and to each other. Then two series of segments or blocks appear, one on each side of the notochord. From these two rows of blocks the vertebrae and muscles will later arise. The sides of the disk are tucked under, and meet, and unitebeneath to form the tubular vertebrate body. Then legs and wings appear, looking for a time like short stubby flippers, in which joints and fingers and toes will arise later. Slowly and gradually the eye is built, the brain shaped, the face moulded. For some time the head is shaped much like that of a lizard. Until far on in embryonic life it is difficult or impossible to tell whether we are looking at the embryo of a bird or of ...