CHAPTER II. 



NATURAL HISTORY AND EXTERNAL CHARACTERS. 



1. The Frog. The common frog, Eana temporaries, is tolerably 

 abundant in summer in damp places and by the side of ponds. 

 In winter it is not readily to be found, for at that season of the 

 year frogs hibernate, often in groups, buried in the mud and 

 under water. The warmth of spring rouses them from their 

 torpor, and they then congregate and pair with much sound of 

 croaking. The female lays a number of eggs, about one-tenth 

 of an inch in diameter, each of which is surrounded by a thin 

 layer of albumen. And as they are laid the male pours 

 upon them a fertilising fluid. If a little of this fluid, or the 

 water into which it is shed, be examined under the micro- 

 scope (high power), a great number of minute active bodies of 

 delicate tapering form will be seen. They are the spermatozoa, 

 the essential elements in fertilisation. After the eggs have been 

 fertilised by the entrance into each egg of a spermatozoon, the 

 albuminous coating of the yolk swells to many times its original 

 thickness by the absorption of water; so that the frog-spawn 

 then has the appearance of a white gelatinous mass, made up of 

 largish jelly 4ike spheres, in the midst of each of which is the 

 dark ovum, which is seen, on closer inspection, to have a darker 

 and a lighter hemisphere. 



In a few hours after fertilisation a groove forms on the darker 

 hemisphere, gradually extends round the egg, and, becoming 

 deeper and deeper until it reaches the centre, cleaves the ovum 

 into two hemispheres. Let us note clearly what has taken place 

 here. The ovum, to begin with, is a single cell, and within it is 

 a specialised portion which would seem to be of special import- 

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