REPRODUCTION 4 1 



animal pole and increases to a maximum at the opposite vegetal pole 



(Fig. 38). The nucleus lies in the animal hemisphere. 



In the eggs of reptiles and birds the quantity of yolk is so enormously 

 greater than that of the protoplasm that the dispersion of the yolk granules 

 throughout the protoplasm would be comparable to dispersing a quart 

 of sand grains in a single drop of water. Accordingly the polarizing of 

 these eggs reaches its maximum in that the protoplasm is all aggregated 

 at one spot on the surface of the egg, thus marking the animal pole, while 

 the remainder of the egg is yolk devoid of protoplasm. The locaUzed 

 protoplasm constitutes the so-called germ disc (Fig. 40) which usually 

 appears as a small white fleck on the surface of the large yellow yolk mass. 

 Before the egg is fertilized the germ disc contains a single nucleus, the 

 original egg nucleus. 



Such a large ovum, consisting, for the most part, of loosely coherent 

 yolk particles, is necessarily invested by a tough vitelline membrane. 

 External to the vitelline membrane are various additional envelopes. 

 Birds and most reptiles, the Uzards and snakes being the exceptions, have 

 a thick layer of albumen (the "white" of a hen's egg) which doubtless 

 afifords the egg some mechanical protection and, in the later stages of 

 development, may be utilized for the nutrition of the young animal. The 

 outermost shell consists of an organic matrix more or less impregnated 

 with salts of calcium. In most reptiles the shell is of a somewhat flexible 

 or leathery texture. In crocodiles and alligators and in birds the shell is 

 highly calcified and consequently hard and brittle. Against the inner 

 surface of the shell lies a thin fibrous shell-membrane. 



It should be appreciated that the object referred to in kitchen and 

 market as an "egg" consists of the actual (in strict sense) egg or ovum, 

 plus various extraneous substances and structures. The hen's ovum, 

 comparable to some small fish egg, is merely the yellow sphere commonly 

 called the "yolk" of the "egg," enclosed in its vitelUne membrane. 



Eggs of Mammals 



In point of size and quantity of yolk, the eggs of birds lie at one 

 extreme. The eggs of mammals, with two exceptions, lie at the other. 

 The exceptions are the duck-bill (Ornithorhynchus) and the spiny anteater 

 (Echidna) of the Australian region. These mammals, presumably of 

 primitive tvpe, exhibit many reptihan features, conspicuous among which 

 is the method of reproduction. Their large eggs contain much yolk, are 

 encased in a tough shell, and are very similar to the eggs of reptiles. With 

 these exceptions, the eggs of mammals contain a minimum of yolk and are 

 of practically microscopic dimensions (0.06 to 0.3 mm. in diameter). The 

 egg (Fig. 33) is covered by a transparent membrane (zona pellucida) 



