STRUCTURE OF THE VERTEBRATES 79 



of preservation of the marsupials in Australia, the student may 

 refer to Chapter XX. 



Among the better known Australian marsupials are the kan- 

 garoos. The student need not be reminded of their small fore 

 feet, their large hind legs and their powerful tails which assist 

 the animals in jumping. The largest stand more than six feet 

 high. The wombat is a herbivorous marsupial resembling a 

 ground-hog. Other herbivorous species are no larger than rats, 

 and much resemble these rodents in external appearance. Other 

 marsupials are carnivorous, the better known being the small 

 Tasmanian Devil and the Tasmanian Wolf. The latter is about 

 the size of the gray wolf, and but for its fundamental marsupial 

 structure could be mistaken for it. However, no matter how 

 closely one of these animals may resemble a higher (placental) 

 animal, it is structurally and developmentally a marsupial. 



Development. The marsupials have small eggs which are al- 

 most without yolk, and of microscopic size. Fertilization is 

 internal, as in all amniotes. The fertilized eggs are retained in 

 the modified posterior part of the oviducts. This portion of tlie 

 ducts becomes the uteri (and later a single uterus) in the two 

 higher groups of mammals. The uterine wall secretes a nutrient 

 fluid, the so-called ''uterine milk", upon which the embryos live 

 by absorption. As there is little or no yolk, and no connection 

 with the mother, the young are born in a very immature con- 

 dition. 



The new born marsupial is merely an embryo with a few well 

 developed, specialized characters. At birth an opossum, \vhich 

 when adult is about the size of a house cat, is not larger than a 

 bean. It weighs approximately four grains; or, to state it more 

 graphically, 1750 of them weigh a pound. A few reflexes are pres- 

 ent, including a negative geotropism, and the young climb up 

 the side of the mother and into the pouch with the aid of front 

 claws which are later shed. There they attach themselves to the 

 nipples, which are long and about the diameter of broom straws. 

 The epithelium of the lips fuses to the nipple and they remain 

 firmly attached for a week or more. By that time they are able 

 to see, and soon begin climbing out of the pouch on to the 

 mother's back. 



