GENERATION, SEX AND ONTOGENY 



225 



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

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

 the bullhead shark is leathery and colored like 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 produced by 

 different species of animals. Among the animals we know 

 familiarly, as the mammals, which give birth to young alive, 



a 





FIG. 132. Eggs of different animals showing variety in external appearance : a, Egg 

 of bird; 6, eggs of toad; c, egg of fish; d, 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 perhaps a few times in a year. The 

 robin lays five or six eggs once or twice a year; a cow may 

 produce a calf each year. Rabbits and pigeons are more 



