THE PRESIDENT'S ADDRESS. 383 



vidual. The great writers of the past, Swammerdam and Reaumur, 

 exercised the greatest ingenuity and patience in the attempt to 

 solve the problem, but the present state of our knowledge is a 

 modern development. It is true that Eathke's beautiful and 

 classical work on the development of the crayfish was published as 

 early as 1829, but Zaddach's paper on the development of the 

 eggs of the Caddis flies, published in Berlin in 1854, is the first 

 great work with which I am acquainted on insect development in 

 the egg. Since that date the literature on this subject has literally 

 flooded us. Brandt in his treatise on the development of the egg 

 gives over 200 references to papers more or less directly connected 

 with the subject in 1878. I will only refer to the latest discovery 

 in the embryology of insects in the egg, because I feel a personal 

 interest in it. 



Until quite recently the formation of the mesenteron or central 

 segment of the alimentary canal was supposed to be quite ex- 

 ceptional in insects. No one had ever seen the invagination of 

 the hypoblast or the blastopore. Last summer I was fortunate 

 enough to cut a longitudinal section of an embryo which showed 

 that the hypoblast is formed by invagination, and I had prepared 

 a paper on the subject when I received the current number of the 

 " Zoologische Anzeiger," and found a note by Biitschli with a 

 figure exactly representing the formation of the mesenteron as I 

 had seen it in my section. 



Although, perhaps, in a morphological sense, the development 

 of the embryo in the egg is the more important, the after-develop- 

 ment of insects in the pupa and more especially that in the fly, is 

 the more interesting, from its very extraordinary and exceptional 

 character. 



Harvey, whom I have already alluded to, made a remarkable 

 generalization with regard to the pupa in insects. He seems to 

 have had an intuition which enabled him to predict modern dis- 

 covery. For him the eggs of insects had not a sufficiency of nutriment, 

 so that the embryo quits the egg in an imperfect state, and when 

 it has obtained a sufficiency, returns to the state of the egg, i.e., 

 the pupa is this second egg in which the perfect insect is developed 

 and in it we find a secondary yelk. 



It was, however, Dr. A. Weismann who first described in detail 

 the manner in which this extraordinary process occurs, in 1863 in 

 his i( After-development of the Diptera." 



