THE EARLY DEVELOPMENT OF THE FROG 69 



to 2.0 mm. in the commoner forms. Externally one-half or 

 more of the egg is colored densely black by the accumulation 

 of pigment granules, while the remainder is a creamy white 

 (Fig. 22, A) ; the relative extent of the light and dark areas may 

 vary -considerably. This is an external indication of the funda- 

 mental polarity of the ovum which is expressed in a variety of 

 ways; the pigmented region is the animal pole. When undis- 

 turbed in the water the eggs tend to assume, very slowly, a 

 definite position such that the animal pole is turned upward. 

 The animal pole is slightly flattened and frequently in this 

 region a, spot (the fovea) of lighter color may be seen. 



The egg is provided with primary, secondary, and tertiary 

 membranes. The primary or vitelline membrane is very thin 

 and delicate, and is often indistinguishable on account of the 

 accumulation of the pigment granules within or under it, and 

 it may never actually separate from the surface of the egg; its 

 existence is denied by some. The secondary membrane, or 

 chorion, is a thin tough membrane, a secretion of the ovarian 

 follicle cells. Surrounding this the tertiary membrane forms 

 a thicker jelly-like layer derived from the walls of the oviduct; 

 just at the time of laying this is rather dense and closely 

 applied to the surface of the chorion, though in a short while 

 it enlarges and then its structure becomes clear (see below). 



The egg proper is composed of different kinds of substance, 

 three of which are visibly differentiated (Fig. 23, C). The 

 pigment granules, which appear brown in a thin layer, are 

 found in small numbers scattered throughout the upper half, 

 or more, of the egg, and on the surface of this they are collected 

 in a thin but dense layer. They thin out rather abruptly just 

 below the equator of the egg. This pigment is apparently to 

 be regarded as a metabolic by-product and is without particular 

 prospective significance in development, although as mentioned 

 later it may absorb heat and so tend to elevate the temperature 

 of the egg. This part of the egg contains also a small amount 

 of deutoplasmic material. The protoplasm of the lower or 

 vegetal pole of the egg is filled with this deutoplasm in the 

 form of granules or semifluid plates, round or oval, and of 



