197 



On the Imaginal Discs of Dr. August Weismann. 

 By B. T. Lowne, M.R.C.S., &c. 



{Read May 28th, 1869.) 



The development of the fly has been the subject of a most elaborate 

 paper by Dr. Weismann, written m 1866 (Kolliker and Siebold 

 Zeitschrift, band. 14). Although the observations of this learned 

 naturalist have long been known in Germany, they seem to have 

 attracted little attention in this country, and some incredulity. The 

 specimen I have brought with me to-night shows the structures 

 from which the head and thorax of the perfect insect is formed, 

 attached to the main nerves, near their origins from the brain of 

 the maggot, which they closely surround. 



The substance of Dr. Weismanu's paper is that the head and 

 thorax of the fly, with the wings and legs, are developed from a 

 number of discs (Imaginal-Scheiben) attached to the main nerves 

 and tracheee of the maggot, which do not coalesce until the third 

 day of the pupa state. The abdomen is developed from cells imme- 

 diately dependent for their formation upon the eight posterior 

 larval segments. Dr. Weismann states his belief that in all those 

 insects in which the anterior larval segments bear legs, the head 

 and thorax also depend for their formation upon the corresponding 

 larval segments ; but in those in which these appendages are absent 

 in the larva, the head and thorax of the perfect insect is entirely 

 dependent for its form and development upon imaginal discs, like 

 those in the larva of the fly. Another most remarkable fact in the 

 development of the fly from the larva, is that all the larval organs 

 undergo degeneration, and that not one of these remain in the 

 perfect fly. Extraordinary and incredible as these observations 

 appear, I am happy to be able to state that I have entirely verified 

 them, and trust that no great time will elapse before I place the 

 result of my observations before the scientific world. In the mean- 

 time I cannot but express my entire concurrence in Dr. Weismann's 

 views. 



