128 THE BOOK OF NATURE STUDY 



enormous egg-cell dilated with yolk, and it of course corre- 

 sponds to the frog's ovum. The difference is that there is 

 relatively a large quantity of yolk in the bird's ovum, and 

 that it is more strictly separate from the small drop of for- 

 mative living matter which lies at the upper pole ; whereas 

 in the frog's ovum there is relatively less yolk, and it is 

 not so definitely separable from the living matter which 

 permeates it. Thus the bird's egg divides partially, the cell 

 divisions being restricted to the minute drop of polar living 

 matter, while the frog's egg divides wholly. The white of egg 

 in a hen's egg is analogous with the envelope of jelly in the frog's 

 egg, and both are produced by the glandular walls of the oviduct 

 as the egg passes down. It is obvious that the frog's egg has no 

 shell. In toad's spawn the eggs lie two or three abreast in two 

 long mucilaginous strings. The comparison should go further, 

 and the frog's egg should be compared \\ith other eggs which 

 can be shown in school, e.g. the eggs of fishes (the familiar roe of 

 cod and herring, for instance), the eggs of the water-snail, the 

 eggs of insects (such as a butterfly's), the eggs of the sea-urchin, 

 and so on. It is useful to make a collection to illustrate the 

 general fact that in all ordinary cases animals develop from 



eggs. 



Practical Note. Samples of the eggs of frogs and toads should 

 be kept for demonstration preserved in alcohol. At the spring 

 season some eggs should be collected and kept in shallow dishes, 

 such as milk-basins, with a few water-weeds to secure aeration 

 and with occasional changes of water, best secured by a gentle 

 drip from the tap. They should not be exposed to the glare of 

 the sun, nor kept too much in the dark. 



Uses of Gelatinous Envelope. The spheres of jelly around 

 the eggs are useful in various ways. They obviate overcrowding 

 and act as buffers, saving the delicate eggs from being jostled 

 by wind and other disturbances. They leave interstices in which 

 small unicellular plants are often seen, along with minute animals 

 such as water-fleas and rotifers. The minute plants liberate 

 oxygen during the day, and absorb the carbon-dioxide given off 

 by the developing eggs. Thus there is a mutually beneficial 

 association, illustrating the " balance of nature " on a minute 



