172 HOUSE, GARDEN, AND FIELD 



duces another kind of egg, distinguished by several 

 peculiarities from the eggs mentioned above, being larger, 

 much darker in colour, lodged in a special protective 

 case, fertilised by the male, and capable of lying dormant 

 for weeks, months, or even years before they develop. 

 These eggs have long been called " winter-eggs," while 

 the smaller, paler, unfertilised eggs, which have no cover- 

 ing except their own shells, have been called " summer- 

 eggs." These names, as we shall soon have occasion to 

 remark, are not altogether appropriate, for winter-eggs 

 may be produced in the height of summer. 



The formation of the outer protective case of the winter- 

 eggs is very singular. It is fashioned out of part of the 

 back of the carapace of a living water-flea. The specialised 

 part becomes dense, dark- coloured, and bordered by a 

 wide strip, which on close examination is seen to be im- 

 pregnated with air, a great number of air-bubbles being 

 lodged each in a separate cavity of the shell. When the 

 time for liberation of the egg-case has come, it splits out- 

 side the air-containing border, and then doubles in two, 

 catching up usually two, but sometimes more than two 

 eggs, which have previously been fertilised and passed 

 out from the ovary. The egg-purse (often called the 

 ephippium, which means saddle) is now found, not merely 

 to be doubled in two, but to possess an inner and an outer 

 wall, separable from each other. The greater part of the 

 carapace is really hollow at all times, its inner and outer 

 walls being separated by a thin space in which before a 

 moult the new carapace forms. Hence when a bit of the 

 two-walled carapace is, so to speak, punched out, it is found 

 to consist of two separate layers, each doubled in two. 

 The walls of the egg-purse are springy, and the edges meet 

 elastically without the help of muscles or any visible ap- 

 pliance, locking up the eggs automatically. The purse 

 never opens again until the young Daphnias developed 

 within it force a passage for themselves. When the egg- 

 purse is set free, it floats away on the top of the water, 



