194 



COMPARATIVE ZOOLOGY. 



spot {cicatricula) on the outside cannot be hardened, even 

 with the most prolonged boiling. The cicatricula, or em- 

 bryo-spot — the part for which all the rest was made — is 

 a thin disk of cellular structure, in which the new life 

 first appears. This was originally a simple cell, but de- 

 velopment has gone some way before the egg is laid. It 

 is always on that side which naturally turns uppermost, 

 for the yolk can turn upon its axis ; it is, therefore, al- 

 ways nearest to the external air and to the Hen's body — 

 two conditions necessary for its development. There is 

 another reason for this polarity of the egg: the lighter 

 and most delicate part of the yolk is collected in its 

 upper part, while the heavy, oily portion remains be- 

 neath. 



In most eggs the shell and albumen are wanting. When 

 the albumen is present, it is commonly covered by a mem- 

 brane only. In Sharks, the envelope is horny; and in 

 Crocodiles it is calcareous, as in Birds. 



The egg of the Sponge has no true vitelline membrane, 

 and is not unlike an ordinary amoeboid cell. An egg is, 

 in fact, little more than a very large 

 cell, of which the germinal vesicle is 

 the nucleus. 



The size of an egg depends mainly 

 upon the quantity of yolk it contains ; 

 and to this is proportioned the grade of 

 development which the embryo attains 

 when it leaves the egg. 108 In the eggs 

 fig. 163.— Egg of sponge: of the Star-fishes, Worms, Insects, Mol- 

 * nucleus. ]ugks ( except the Cuttle-fishes), many 



Amphibians, and Mammals, the yolk is very minute and 

 formative, i. e., it is converted into the parts of the future 

 embryo. In the eggs of Lobsters, Crabs, Spiders, Cepha- 

 lopods, Fishes, Reptiles, and Birds, the yolk is large and 

 colored, and consists of two parts — the formative, or 



