GENERATION, SEX AND ONTOGENY 



225 



imbedded in a soft, jellylikc substance; the skate's egg has a 

 tough, dark-brown leathery inclosing wall; tlie spiral egg of 

 the bullhead shark is leathery and colored hke the dark-olive 

 seaweeds among which it lies; and a bird's egg has a hard 

 shell of carbonate of lime. But in each case there is the essen- 

 tial fertilized germ cell; in this the eggs of hen and fish and 

 butterfly and crayfish and worm are alike, however much they 

 may differ in size and external appearance. 



There is great variation in the number of young ])roduccd by 

 different species of animals. Among the animals we know 

 familiarly, as the mammals, which give birtli to young ahve, 





Fig, 132. — Eggs of different animals shov.ing variety in cxternaJ api>earanco : a, Egg 

 of bird; h, eggs of toad; c, egg of fish; rf.egg of butterfly; e, eggs of katydid on leaf; 

 /, egg case of skate. 



and the birds, which lay eggs, it is the general rule that but 

 few young are produced at a time, and the young are born or 

 eggs are laid only once or perhaj^s a few ti nes in a year. The 

 robin la^^s five or six eggs once or twice a year; a cow may 

 produce a C^lf each year. Rabbits ai J i>igeori? are more 



