430 



How Aimuals and Pla?its Reproduce unit viii 



large numbers of eggs fertilized at one 

 time, depending partly on how many 

 eggs have entered the oviduct. In many 

 kinds of animals a protective covering 

 promptly forms around the fertilized 

 e.^^ or eggs, which are then laid. 



Among many kinds of animals inter- 

 esting examples of instinctive behavior 

 are shown in the care of these fertilized 

 eggs. The crayfish may carry the eggs 

 about with her, suspended from the ab- 

 domen. Insects often lay their eggs 

 where they will escape observation. The 

 female grasshopper uses a special organ 

 at the tip of the abdomen to dig a hole 

 for each tg^^ which remains well pro- 

 tected against enemies and cold. Flies 

 find various kinds of decaying material 

 in which to deposit their eggs; when 

 they hatch, the young are near suitable 

 food. The ichneumon fly starts her 

 brood in the body of a particular cater- 

 pillar which will later serve as food for 

 her young. In another kind of insect 

 which also lays eggs in the bodies of 

 large caterpillars, the larvae later chew 

 out of the caterpillar's bod\' and spin co- 

 coons on the outside. See Figure 380, 



Reproduction in birds. Only one of the 

 two ovaries develops in birds. The ovary 

 of the hen is readily seen when the or- 



FiG. 380 (above) Where did the larvae of these 

 wasp cocoons get foodFi am. mus. of nat. hist.) 



Fig. 379 (left) The male tnidwife toad carries 

 the eggs abo7it with hi?}! until they hatch into 

 tadpoles, (am. mus. of nat. hist.) 



gans of a freshly killed chicken are re- 

 moved. See Exercise 7. There are egg 

 cells of many sizes in this ovary. As each 

 egg cell reaches its full size, it leaves the 

 ovary and passes into the oviduct. You 

 know the egg cell as the yolk of the hen's 

 egg. If the male bird has previously in- 

 troduced sperm cells into the oviduct, 

 they have travelled up the duct to a 

 pouch near the upper end. As the egg 

 cell enters the oviduct it is fertilized by 

 one of these sperm cells. The fertilized 

 egg cell then travels along the oviduct 

 on its way to the exterior. In passing, it 

 is \\Tapped in protective coverings of 

 various kinds. Study an egg by doing 

 Exercise 8. It is surrounded first by the 

 white of egg (albumen). This not only 

 helps to protect the developing embryo 

 but later serves as food for it. Further 

 along, the oviduct secretes two thin but 

 tou<Th membranes around the cga and 

 lastly a hard shell. 



In the meantime the fertilized eg-a has 

 begun to divide; but by far the greater 

 part of the development occurs after the 

 egg has left the body of the mother. This 

 development does not take place unless 

 the egg is kept warm. Generally the bird 

 sits on the eggs during an incubation 

 period, which lasts about three weeks. 



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