156 Embryonic Development of Chironomiis 



Facilities 

 for sti^dy. 



Writers 

 on the 

 develop- 

 ment ol' 

 Chiro- 

 nomns. 



The cord is so tough, that it can be stretched with a pair 

 of needles without breaking. A steady pull at the ends 

 of the cylinder will draw it out to nearly twice its original 

 length without injury ; if stretched beyond this point, the 

 cord becomes strained, and does not perfectly recover its 

 shape when released. The ends of the cord pass into an 

 adhesive disk, which is attached at the surface of the 

 water. Thus the whole mass, containing hundreds of 

 eggs, is firmly moored, yet so moored that it floats with- 

 out strain^, and rises or falls with the level of the water. 

 The eggs get all the sun and air which they require, and 

 neither predatory insects, nor birds, nor water-moulds, 

 nor rushing currents can injure them. 



There are few animals which afford greater facilities 

 than Chironomus for the study of embryonic development. 

 The eggs are very plentiful, and can always be had during 

 the summer months ; they are so transparent as to admit 

 of the use of fairly high powers of the microscope ; and 

 since they require no preparation for study as whole 

 objects, they can be replaced in water after inspection, to 

 continue their development. This diminishes the diffi- 

 culty of ascertaining the order of the developmental 

 chan2:es, which is nevertheless considerable, even in Chi- 

 ronomus. * The development is com^^leted in six days or 

 less, so that every part of the process can be observed 

 without prolonged waiting. On the other hand, the small 

 size of the eggs is a serious difficulty in the preparation 

 of sections, and in some important respects the develop- 

 ment is peculiar, and not typical of insects. 



"We have now three excellent accounts of the embry- 

 onic development of Chironomus. AVeismann (1863) 

 studied living or fresh embryos for his classical memoir. 

 It was largely upon facts drawn from the development 

 of Chironomus that he long afterwards based his theory 

 of the Continuity of the Germ-plasm (1889). At the time 



