198 PRINCIPLES OF ANIMAL BIOLOGY 



of insuring fertilization so there is apparently none in the method of 

 bearing the young. Oviparity and viviparity are found in the verte- 

 brates and the invertebrates. Certain conditions of the reproduction 

 itself, however, make one generalization possible. It is to be noted that 

 the forms in which the eggs arc fertilized odtside the body of the mother 

 are necessarily oviparous. Also, it is only among forms with internal 

 fertilization that viviparity, ovoviviparity and the laying of fertilized eggs 

 can occur. It results that viviparity, ovoviviparity and the laying of 

 fertilized eggs prevail among land forms, where protection against evapo- 

 ration of the eggs is necessary, and that the habit of laying eggs before 

 fertilization is mostly found among the aquatic species and the am- 

 phibious forms which lay their eggs in water. 



Care of Fertilized Eggs. — Among oviparous species the methods of 

 caring for the fertilized eggs are almost endlessly varied in their details. 

 There are many animals which give no care whatever to the eggs. This 

 is particularly true of aquatic species which pour the eggs and sperms 

 freely into the water to come together by chance. The starfishes and 

 sea-urchins and many other marine animals exhibit this lack of any 

 parental care. Other forms merely put the eggs in places where develop- 

 ment is facilitated. Thus, toads and certain salamanders which live 

 on land in the adult stage lay the eggs in the water. The aquatic turtles 

 come to land to lay the eggs in the warm sand which hastens their develop- 

 ment. The digger wasps, ichneumon flies and certain other insects de- 

 posit their eggs in various places and provision them with living or dead 

 animal food. One group of birds, the Megapodes, lay the eggs in a pile 

 of decaying vegetation, the decomposition of which liberates heat that 

 aids in development (Fig. 151). Again manj'' animals build nests. These 

 nests may be very simple in construction. In the fishes, for example, 

 many species merely hollow out a small area on the bottom of the 

 stream by pulling out the pebbles and heaping them up on the down- 

 stream side of the nest. The eggs, when laid, drop into this hollow and 

 among the loose stones. Birds build nests of a great variety of forms, 

 from the loose collection of grass or straw put on the ground by the kill- 

 deer, or the insecure litter of twigs set in the branches of trees by the 

 mourning dove, to the elaborate hanging-basket of the orioles. Still 

 other forms enclose their eggs in cases, as was pointed out for the earth- 

 worm in the preceding chapter, and as is true also of the leeches and some 

 insects, snails, and spiders. 



Among the nest building forms the habit of caring for the eggs has 

 usually been developed, that is, one or both of the parents in many species 

 remain with the eggs until they are hatched. The habit of remaining with 

 the eggs may insure incubation, or the elevation of the temperature to a 

 point at which development will proceed. Incubation by the parents is 

 necessary in most birds, and is an aid in some other animals. Remaining 



