NOTES 481 



168 The thigh bone ossifies from five centers. The bone eventually 

 unites to one piece. 



169 jr or this reason, mammals are called viviparous ; but, strictly 

 speaking, they are as oviparous as birds. The process of reproduction is 

 the same, whether the egg is hatched within the parent or without. The 

 eggs of birds contain whatever is wanted for the development of the em- 

 bryo, except heat, which must come from without. Mammals, having no 

 food yolk, obtain their nutrition from the blood of the parent, and after 

 birth from milk. 



170 The larvae of butterflies and moths are called caterpillars ; those of 

 beetles, grubs ; those of flies, maggots ; those of mosquitoes, -wigglers. 

 The terms larva, pupa, and imago are relative only ; for, while the grub 

 and caterpillar are quite different from the pupa, the bee state is reached 

 by a very gradual change of form, so that it is difficult to say where the 

 pupa ends and the imago begins. In fact, a large number of insects reach 

 maturity through an indefinite number of slight changes. The bumblebee 

 moults at least ten times before arriving at the winged state. 



171 Every tissue of the larva disappears before the development of the 

 new tissues of the imago is commenced. The organs do not change from 

 one into the other, but the new set is developed out of formless matter. 

 The pupa of the moth is protected by a silken cocoon, the spinning of 

 which was the last act of the larva ; that of the butterfly is simply inclosed 

 in the dried skin of the larva, which is called chrysalis because of the 

 golden spots with which it is sometimes marked. The pupa of the honey- 

 bee is called nymph ; it is kept in a wax cell lined with silk, which the 

 larva spins. The time required to pass from the egg to the imago varies 

 greatly ; the bee consumes less than twenty days, while the cicada requires 

 seventeen years. 



172 Compare the amount of food required in proportion to the bulk of 

 the body, and also with the amount of work done, in youth, manhood, and 

 old age. 



173 Excepting, perhaps, that the new tail of a lizard is cartilaginous. 



174 The patella, or kneepan, has no representative in the adult fore 

 limb. 



no The structure of the highest plants is more complex than is that of 

 the lowest animals ; but, for all that, powers are possessed by jellyfishes of 

 which oaks and cedars are devoid." MIVART. 



176 It is, however, true that the number of eggs laid is proportioned to 

 the risk in development. 



177 See Lewes's charming " Studies of Animal Life." Doubtless an 

 examination of all the strata of the earth's crust would disclose forms 

 immensely outnumbering all those at present known. And even had we 

 every fossil, we should have but a fraction of the whole, for many deposits 

 have been so altered by heat that all traces have been wiped out. Animal 

 DODGE'S GEN. ZOOL. 3 i 



