384 the president's address. 



When we remember the primitive modes of histological research 

 which were then in use, and the unsettled state of men's minds with 

 regard to the manner in which the various tissues are developed, we 

 must regard this paper of Weismann's as one of the most wonder- 

 ful, if not the most wonderful, record of brilliant discovery which 

 has appeared in our lifetime. The whole of it reads almost like a 

 fairy tale, and yet, from my own personal work, I can verify all 

 the main facts as set forth by this great German biologist. Swam- 

 merdam and Reaumur observed certain swellings on the nerves of the 

 dipterous larva which they believed to be ganglia. Weismann dis- 

 covered that these are the germs from which the imago is developed, 

 and observed the manner in which they unfold and become united 

 to each other during the development of the nymph within the pupa 

 case. 



Weismann made mistakes undoubtedly, but these do not detract 

 from the greatness of his discoveries. It would be as reasonable 

 to take the credit of the discovery of America from Columbus, 

 because he supposed it to be a part of Asia, as to detract from 

 Weismann's masterly work because the discoveries of the last 

 twenty years were not all made by him. 



Weismann named the rudimentary structures from which the 

 perfect fly is developed " imaginal discs." He traced the develop- 

 ment of the head from two such discs and that of the thorax from 

 twelve. He believed that the abdomen is formed from that of the 

 larva ; and supposed that the tissues of the fly are developed from 

 the ruins of those of the larva, by a process of free cell formation. 



This process, which has some analogy with the spontaneous 

 generation, is certainly a myth, but Weismann had no means of 

 accurately examining the discs — section cutting was unknown — 

 and he made his observations by rendering the tissues transparent 

 with various reagents. So thick were the masses on which he 

 operated that, like Swammerdam, he had recourse to direct sun- 

 light for the illumination of his objects, and so persistent was his 

 work that I am informed he nearly blinded himself. 



It was Ganin who first determined the fact, unknown to Weis- 

 mann, that these imaginal discs consist of mesoblast as well as 

 epiblast, and that not only the integument, but all the tissues of 

 the perfect insect, originate from them. 



Ganin also discovered the fact, that the abdomen of the perfect 

 insect, as well as the alimentary canal, is developed like the head 



