I3& SCIENCE EROM AN EASY CHAIR 



The young snails nearest the opening to the exterior are 

 the furthest advanced in development, and are as big 

 as a dried pea. All stages, from the minute germ just 

 fertilised to well-formed young, may be found in these 

 snails, and the whole course of their development and 

 gradual change and growth can be minutely studied with 

 the microscope in one specimen. 



Similar devices for protecting the young in their 

 earliest helpless stages of growth from the egg-shell are 

 found in all classes of animals. What is very curious is 

 the fact that, of two closely allied animals, one species 

 will recklessly lay its eggs and leave them, whilst 

 another has special arrangements for retaining in the 

 parent's body the eggs as they develop, and so pre- 

 serving them from danger. Such parents are called 

 " viviparous." Of course, in all viviparous animals, as 

 well as in those which lay their eggs in hard shells, the 

 fertilisation of the egg must be effected within the 

 maternal body. Amongst our common fishes there is 

 the viviparous blenny, often found in pools at low tide 

 on the seashore. All the other British fresh-water 

 and marine fishes lay their eggs and abandon them, 

 excepting some sharks, dog-fish, and skates, which are 

 viviparous ; others of the shark and skate tribe lay eggs 

 of large size encased in hard, horny shells. Every one 

 knows that frogs and toads lay their eggs, but there are 

 some kinds in which the eggs remain inside the mother's 

 body during the development of the young, which only 

 escape into the world as well-formed little frogs. All 

 the hairy, warm-blooded quadrupeds known as " the 

 mammals " are viviparous, except the duck-mole and 

 the spiny ant-eater of Australia. These extraordinary 

 little " beasts " lay eggs like those of a bird. 



The most ingenious devices for the protection of the 

 young are (as perhaps those who believe in the superior 



