Chap. 19 DEVELOPMENT 361 



of a fertilized hen's egg is changed to the liveliness of a chick that can aim a 

 peck at another chick's bright eye and strike it. 



The Yolk Content of Eggs — Its Food Value and EflFcct on Development. 



Because of their content of yolk, eggs are the largest cells in the body. Even 

 in the eggs of mammals a minute amount of yolk is present, a fragment of his- 

 tory from their egg-laying ancestors. The egg cell of a mouse (0.06 mm. in 

 diameter) is one of the smallest eggs of vertebrates; those of some of the huge 

 sharks are the largest eggs known. The egg of an ostrich (85 mm. in diameter) 

 is the largest of any familiar animal. It weighs three pounds and contains the 

 equivalent of one and a half dozen hen's eggs. The ancient birds produced the 

 really large eggs with enough food for a banquet in one yolk. The fossil egg 

 shell of the extinct bird Aepyornis holds a gallon. 



Except in mammals, yolk is the complete food for embryos. Its value as 

 human food has greatly added to the economic importance of hens, ducks, and 

 ostriches. The eggs of fishes have not only food value tut, in caviar, they add 

 social prestige. It is the yolk that counts; "fried eggs" mean yolky hens' eggs, 

 never cows' eggs. The high value of yolk is due to the completeness of its food 

 content of proteins, fats, carbohydrates, inorganic salts, vitamins, pigments 

 (carotin in birds), and enzymes; water composes about half of its bulk. 



Yolk changes the pattern of development because it takes no part in cell 

 division except as it is a hindrance. Obviously, there must be less protoplasm 

 in parts of the cell that are packed with yolk; there cell division is slow because 

 the rate of metabolism is low. Cell division must combat the inertia of yolk or 

 avoid it by taking a roundabout way, as it does in the early embryos of frogs, 

 birds and many other animals (Figs. 19.1, 19.7). Yolk accumulates in one 

 hemisphere of the egg of frogs, and forces the nucleus into the other. Since 

 yolk is heavier than protoplasm, the vegetal pole where it is most abundant is 

 always down and the lighter animal pole is up. 



In large-yolked eggs such as those of the frog and chick, the accumulation 

 of yolk in one region is so great that they are known as telolecithal, "end- 

 yolked" eggs (Table 19.1). In small-yolked eggs, like those of amphioxus and 

 man, the yolk is generally distributed and they are called isolecithal, "equal- 

 yolked." Even in these, there is a visible difference between the poles. 



Fertilization — The Prelude to Development. The main steps in fertilization 

 are the entrance of the sperm into the egg, and the union of the male and 

 female nuclei (Fig. 19.2). The sperm makes its way into the egg membrane 

 stimulating the rise of a cone of protoplasm that surrounds it and draws it into 

 the egg. At the same time, a thin layer of protoplasm, the fertilization mem- 

 brane, is suddenly lifted from the surface of the egg and shuts out other sperm 

 cells. The male and female nuclei, each with half the number of chromosomes 

 to be contained in the body cells of the embryo, now approach one another 

 and come in contact. The first cell division of the new individual follows at 



