254 the president's address. 



which are vegetable and which are animal. Turn to the study oi 

 development, the whole of which is in a progressive state. We are 

 now carrying it so far that we can trace back a single group of 

 organs to a particular portion of the dividing yolk mass, and the 

 ultimate result will probably be to trace out each group of organs to 

 the blastomeres from which it has proceeded. Thirty years ago if 

 any one had talked of making sections of a butterfly's or a crayfish's 

 embryo, it would have been regarded as something inconceivable ; 

 but now careful section makers will deal with these things in such a 

 way as to be able to make you twenty sections through one of them. 

 But as every one knows, this kind of thing cannot be followed out 

 without a great expenditure of time and an amount of dexterity 

 which it is not given to every one to attain. 



Let me also point out that it is not necessary to go far a-field to 

 find subjects for this kind of work ; probably what is most wanted 

 is an exhaustive study of some of the commonest things about us. 

 The development of Rotifers, for example ; why these are the 

 commonest things in the world. Kowalewsky, years ago, made a 

 number of observations upon the common earth worm ; let us see if 

 in his conclusions he was right or wrong. It is not very difficult to 

 make sections of these things, and it would be of great importance 

 to get his observations confirmed. I have no doubt as to their 

 correctness myself, but it would be very satisfactory to have them 

 verified. 1 might turn to half-a-dozen kinds of similar observations 

 with regard to the development of insects — say the presence of an 

 amnion in some and not in others ; any one who could make minute 

 sections would solve the problem for us. It happens that some 

 time ago I occupied myself with a very common subject, by way of 

 carrying outmy own precept of trying to get to the bottom of some- 

 thing. I had various reasons for doing this with the common cray- 

 fish, and I may say that the result has been very instructive ; I 

 mention it to show how much it is worth while to take up common 

 things, if only to find out points about them which have been over- 

 looked. The first thing I did was to examine the branchiaa, and in 

 so doing I found to my surprise that there were two little filaments, 

 about -i- of an inch long, attached to the sides of the body. I had 

 never seen any notice of these things, but I found by-and-bye that 

 Brandt and Ratzeburg had seen them. The curious thing about 

 them was, that these two little filaments occupied the same position 

 as gills, and from this I had little reason to doubt that they were 



