50 General Part. 



is occasionally so far advanced among oviparous animals^ tliat tlie 

 young one Hatclies almost as soon as tlie egg is laid ; wtilst there 

 are others^ whose offspring break the shell at the moment of birth. 



Those eggs which are laid without other covering than a vitelline- 

 membrane or egg-shell, develop without receiving any nutriment 

 from without. Oxygen is absorbed from the environment, and 

 carbon-dioxide is given off, for the egg breathes just as does the 

 perfect animal, and cannot develop in an atmosphere devoid of 

 oxygen. "Water also is sometimes absorbed. When the egg is laid, 

 surrounded by albumen (as in Birds), this is gradually used up 

 during the development of the young organism*, so that whilst it 

 is still enclosed in the shell, it receives an addition to the majterial 

 originally present in the egg-cell. In many, but by no means all, 

 viviparous animals, the developing organism receives food from 

 the parent ; either certain glands opening into the uterus secrete 

 a nutritive liquid which is absorbed by the offspring, or part of 

 the embryo forms aju absorptive organ which comes into intimate 

 relation with the uterine wall, and the young animal lives like a 

 parasite on the blood of the parent (Mammalia). 



So long as the animal remains enclosed in the egg-membranes, or 

 in the body of the parent, it is called an embryo (foetus) . After 

 birth, i.e., when it breaks through the egg-membranes or leaves the 

 parent, as a rule, it begins instantly to lead an independent life, 

 feeding on its own account. t The new-born animal differs 

 more or less from the perfect organism. In some cases, the 

 distinction is relatively unimportant, as when it consists essentially 

 in smaller size and immature genitalia, as in many Mammalia. 

 In other cases the difference is greater, e.g., among Birds; fledglings, 

 as is well known, differ considerably in plumage from the adult. 

 Still more important is this difference in many other animals which 

 are said to undergo a metamorphosis before they attain the 

 perfect form. The changes undergone by the young animals, in 

 such cases called larvae, are often very considerable, and frequently 

 the larva has but the slightest resemblance to the adult. 



The causes and also the nature of differences between larva and 

 adult are of various kinds. In many cases the most obvious is that 

 the egg is too small for a creature like the adult to be formed from it. 

 The imperfect state in which many Fish are hatched is fully accounted 

 for by the small size of the eggs [see the paragraph on the develop- 

 ment of the Pike in the special part, " Pisces ") : in allied forms with 



* This absorption goes ou usually througli tlie whole surface, or by only a part of 

 it. Occasionally the albumen is taken up into the digestive tract of the young 

 organism. 



+ Sometimes there is a greater or less quantity of yolk within the body of the 

 newly born animal, and then it does not feed for a long time. (See also, remarks, p. 52, 

 on " Parental instinct.") 



