ii FLIES WITH AQUATIC LARVAE 151 



the ease with which the development of the larva 

 within the egg can be studied. In the first place, they 

 can always be procured during the summer months. 

 They are so transparent as to admit of examination 

 under high powers of the microscope as living objects, 

 and as they require no sort of preparation, they may 

 be replaced in the water after each examination to 

 continue their development. This saves all trouble in 

 determining the succession of the different stages a 

 point which usually presents difficulties to the embry- 

 ologist. The whole development of the egg of 

 Chironomus is completed in a few days (three to six, 

 according to temperature), and it is therefore an easy 

 matter to follow the process throughout with the help 

 of three or four chains of eggs. 



The development of any complex animal is a diffi- 

 cult study, and involves as a first requisite a good 

 anatomical knowledge of the adult organs, but even 

 a beginner can learn something from the study of 

 living eggs of Chironomus. In an early stage he will 

 see a transparent outer layer of cells, completely 

 inclosing the granular yolk. 1 A little later the outer 

 layer thins away in one place, and in the end exposes 

 the yolk at that point, cutting off what will ultimately 

 be the head of the larva from the tail. Then the 

 transparent body becomes segmented, and the rudi- 

 ments of the antennae and jaws begin to appear. The 

 integument is pushed in at the head-end, so as to 

 form a long narrow tube which reaches up to the yolk. 

 This tube gives rise to the mouth and oesophagus. 

 A similar pushing-in of the integument at the tail- 

 1 See Fig. 44, F., left-hand embryo. 



