550 THE EMBRYOLOGY OF THE FROG 



as though dozens of hen's eggs were broken, but with the yolks entire. 

 The eggs vary in different species from one and five-tenths millimeters 

 in diameter to twice that size. A little over half of the egg is a dense 

 black due to the pigment granules contained therein, while the remain- 

 der is rather white, although again, in different species the quantity of 

 pigment may vary greatly. The darker portion is commonly known as 

 the animal pole, and the lighter as the vegetal pole. 



There are three membranes covering the egg: Primary. That 

 known as the vitelline membrane. This can sometimes be distinguished 

 from the pigmented substance lying directly beneath it, although some 

 writers deny that it exists at all. 



The secondary membrane (sometimes called the chorion) is a rather 

 thin but tough layer secreted from the follicle cells of the ovaries. 



The tertiary membrane is a thick, jelly-like layer derived from the 

 walls of the oviduct, lying close to the chorion, first as a dense layer, 

 #ut later as it enlarges it becomes quite clear. 



It will be remembered that the yolk granules were quite evenly dis- 

 tributed in the yolk of the hen's egg and that the embryo developed upon 

 the yolk. In the frog, however, the deutoplasm or food part of the yolk 

 all lies at one end the vegetal pole. Frog's eggs are therefore said to 

 be telolecithal. 



The nucleus lies in the animal pole and has already commenced to 

 divide by the time the egg is laid. In fact, it is already in the metaphase 

 of the second polar division at that time. The first polar body has been 

 thrown out, and can be seen as a very tiny light spot in the flattened 

 area of the upper pole. 



As the reproductive organs of the adult have just been reviewed, 

 we shall not again discuss them here, but the development of these or- 

 gans will be taken up individually at a little later period. The general 

 development of egg and sperm are quite like that which occurs in the 

 germ cells of the chick. 



FERTILIZATION 



The sperm drills its way through the thin jelly of the chorion and 

 normally enters the egg substance in the pigmented region. The point 

 of entry is a meridian, passing through both poles of the egg. The 

 meridian which passes through the animal and vegetal poles of the egg, 

 as well as through the point where the sperm enters, is called the fertili- 

 zation meridian. Only one sperm norma-lly enters the egg. Polyspermy 

 is not, however, rare, but as far as we know, always results in some ab- 

 normal development when it takes place. 



It will be remembered that after one-half of the chromosome mate- 

 rial of the nucleus of the egg has been thrown out by the two polar 

 divisions, the nucleus which then contains one-half the normal number 

 of chromosomes is called the female pro-nucleus. The head of the sperm 



