428 THE MAMMALS 



in the eggs, so mammal eggs are much smaller than those of other verte- 

 brates. A rabbit egg is no larger than a clot made with the point of a 

 fine-pointed pencil. This is true viviparous reproduction, with the young 

 being born alive, in contrast to the oviparous reproduction found in most 

 other animals where eggs are laid. There are some egg-laying animals 

 whose eggs hatch before being laid, so that the young are born alive. 

 This is sometimes called ovoviviparous to distinguish it from the true 

 viviparous method where the embryo receives nourishment from the 

 mother's blood during development. 



Highly Developed Cerebrum. Whereas the birds concentrated on a 

 highly developed cerebellum, for control of equilibrium and muscle co- 

 ordination necessary for flight, the mammals have developed the cere- 

 brum to the greatest size and complexity found in the vertebrates. Since 

 the cerebrum is concerned with the process of learning and reasoning, 

 the mammals can probably be considered the smartest animals on the 

 earth. The cerebrum has complex folds in the higher mammals which 

 increase its efficiency, for the outer gray matter of the cerebrum is the 

 functional part and when the outer part folds inward the amount of gray 

 matter is greater than it would be otherwise. Man has by far the largest 

 cerebrum and it bears more convolutions in proportion to his size than 

 that of any other mammal. 



No Cloaca. The amphibians, reptiles, and birds have a cloaca, which 

 is a cavity at the terminal end of the intestine into which the wastes of 

 digestion as well as the urogenital products are emptied. Thus, the anus 

 serves as the external opening for all of these. In the embryonic mam- 

 mals a cloaca is formed, but tissue grows between the anus and the open- 

 ings of the urogenital system, so that there are separate openings in the 

 adult. The first order of mammals is an an exception, however, and 

 does possess a cloaca as an adult. 



Order — Monotremata 



This order includes the most primitive of the mammals. They have 

 hair on their bodies, mammary glands, and other characteristics that 

 cause them to be classified as true mammals, but they have a cloaca and 

 lay eggs like the reptiles and birds. 



The little duckbill of Australia is one of the few examples of this in- 

 teresting order of mammals. It has a bill like a duck, fur like a seal, 

 webbed feet like a frog, and barks like a dog ; it seems to be made up of 

 odd parts of other animals. The female lays eggs which have shells like 

 birds' eggs. She makes an underground nest lined with dried grass and 

 the young hatch out after a short period of incubation. There are no 



