REPRODUCTION IN PLANTS AND ANIMALS 369 



378. Reproduction in birds and mammals. In most of the 

 higher animals the fertilized egg proceeds, immediately after 

 fertilization, to the development of the beginning stages, or 

 embryo, of the adult animal. The exception to the last state- 

 ment is found in the case of those animals which lay an egg 

 that is incased in a more or less heavy coating, like the eggs 

 of birds and turtles, in which the real egg is a cell similar 

 to the one already described. In these cases there is, in ad- 

 dition to the real egg, a large amount of food material, all 

 of which is incased in the covering. In turtles this covering 

 is~ a thick, tough, leathery wall; in birds it is thick and frag- 

 ile, though it varies in its strength in different species of 

 birds. The egg of the bird undergoes a period of incubation 

 ranging from fourteen days (or even fewer), in the case of the 

 smaller birds, to four weeks or more, in the case of turkeys, 

 geese, etc. It is possible to discover the stages in the de- 

 velopment of the embryo by examining eggs in various 

 stages of incubation. It will be seen that at first the small 

 embryo is reddish in color and that radiating lines of blood 

 vessels extend from it into the food material. The develop- 

 ing embryo soon differentiates bone, muscle, nerve, and other 

 tissues. In a short time the organs that are recognized in 

 the adult bird may be found completely differentiated in the 

 embryo that is still retained within the egg wall. It is ap- 

 parent, therefore, as already noted, that a development of the 

 embryo within the egg cell has many features in common 

 with the development of the frog. 



In the higher animals, known as mammals, and in some 

 others the egg develops into the young animal while still 

 within the body of the mother. In the reproduction of these 

 animals the new individuals are bom with their various 

 structures already well formed. 



379. Reproduction in seed plants; the flower. Reproduc- 

 tion in seed plants is in some respects quite similar and in 

 others quite dissimilar to that of the animals. The flower 



