THE HARLEQUIN-FLY 121 



neither insects, nor birds, nor water-moulds, nor sudden rushes 

 of water can injure them. 



The eggs of the harlequin-fly are beautifully suited to the 

 wants of the student of development. They are so transparent 

 that they can be studied alive under the microscope. When 

 you have examined them, and noted what progress has been 

 made, you can put them back into the water to develop for 

 another three hours, and three hours is enough to produce a 

 visible change. The whole course of the development is com- 

 pleted in five or six days, so that it is not difficult to observe 

 every important step. 



The egg is long, narrow, and rounded at each end like 

 a bird's egg. It has a tough shell and a thin shell-membrane. 

 Not long after the egg is laid, the cells, out of which the 

 body is to be formed, appear. They creep out of the oily 

 yolk which occupies the interior, and arrange themselves in 

 a regular sheet just within the shell-membrane. The outside 

 of the body forms first, as it does in most animals, and 

 there is an early stage when the body of the future blood- 

 worm is nothing but a hollow bladder, composed of transparent 

 cells and filled with yolk. In this stage there are no organs, 

 and you cannot distinguish head from tail. Very early, how- 

 ever, the layer of cells becomes folded inwards at one place ; 

 the cells infolded soon cease to grow, while the rest grow 

 rapidly. This fold marks the place of separation of the 

 head and tail of the future larva, and for a time there is 

 nothing else to distinguish them. The original single -layer 

 of cells has now thickened, and become divided into 

 numerous layers. Then cross-folds appear, and the body 

 becomes ringed. The jaws and legs are next developed. 

 The nervous system appears early, the digestive tube rather 

 late. At last the body lengthens so much that it becomes 

 too long for the egg-shell, and has to be coiled. Then it 

 begins to move about, and on the fifth or sixth day escapes 

 from the egg as a minute larva with colourless blood and 

 no gills, but otherwise like a blood-worm. 



Further details respecting the development of the larva 

 within the egg are given in the next chapter. 



