REPRODUCTION 223 



but the ducts open at the bases of the appendages of the thir- 

 teenth segment. 



504. The eggs are fertilized at the moment of their escape 

 from the oviducts and are then cemented to the hairs of the ab- 

 dominal appendages of the female. In this way they are pro- 

 tected from other animals; care is taken that they have the 

 necessary supply of aerated water and they are not carried 

 away by currents of water toward the sea. Even after the young 

 are hatched they continue to cling for some time to the appen- 

 dages of the female. 



505. It happens that development is direct in the case of 

 the crayfish, though in many Crustacea there is a well marked 

 metamorphosis. In some Insects development is also direct, 

 as, e. g., in the grasshopper, but in several orders of Insects 

 there is a complete metamorphosis. From the egg of the butter- 

 fly is hatched a small caterpillar. This grows into a large cater- 

 pillar. Then a metamorphosis occurs. The caterpillar be- 

 comes a quiescent pupa and remains such for a time; then an- 

 other change gives birth to the imago butterfly. 



506. A few fishes are hermaphroditic. In all other Verte- 

 brates the sexes are distinct. The gonads are developed from 

 the mesoderm. They have no ducts primarily, but certain 

 tubules which belong primarily to the excretory system become 

 specially modified and assume the function of genital ducts. 

 In some aquatic Vertebrates the eggs are fertilized in the water, 

 but in all reptiles, birds and mammals fertilization takes place 

 in the oviduct and development begins before the escape of the 

 egg from the body of the parent. In the highest Mammals 

 this intra-uterine development continues for weeks, months or 

 even, in the case of the elephant, to nearly two years. In some 

 fishes, and especially in frogs and toads, there is a marked meta- 

 morphosis, but in the higher groups the development is direct. 



