with special Reference to the Mode of Connexion of Cells. 101 



in 18S3 by a second method, which was a modification of that of 

 Hanstein, and consisted in treating sections of fresh material with 

 iodine solution, swelling with Chlor. Zinc. Iod. and staining with 

 picric Hofmann's blue or methyl violet. Certain results obtained by 

 this latter treatment were so promising that in my final paper in 

 1SS3, with the customary rashness of youth, I described the method 

 as being "perfectly satisfactory ;" but no long period elapsed before 

 I found in practice that it was of but limited application. 



Speaking generally, and excepting Poirault's* researches, I think 

 one is justified in saying that since 1883 little or no advance has been 

 made in the improvement of methods, and that later observations 

 rest mainly on small modifications of the methods of Russow and 

 myself. 



The careful and detailed work of Kienitz-Gerlofff unfortunately 

 serves in great measure to demonstrate the unsatisfactory nature of 

 the results obtained by the sulphuric acid method, and to prove that 

 unless the threads are of exceptional size, as in Viscum album, or as 

 in sieve- tubes, the method is unreliable. The above remarks equally 

 apply to such of -my own results as depend upon the sulphuric acid 

 modification. 



An advance was, however, made by Poirault. In place of experi- 

 menting on sections of fresh tissue, he killed and hardened pieces of 

 tissue in dilute iodine solution, and from the preserved material he 

 then cut sections, which were swollen with Chlor. Zinc. Iod., or 

 sulphuric acid, and stained either with eosin, Poirrier's acid brown, 

 methyl violet, crocein, or aniline green. Poirault's researches are 

 limited to the ferns and other vascular cryptogams, and lie buried, 

 so to speak, in his paper " Anatomical Researches on the Vascular 

 Cryptogams." While certain of his figures are, perhaps, not entirely 

 convincing, the results of his research are most important, and of 

 great interest. I am ashamed to say that I was unaware of the 

 existence of this paper until the autumn of 1895, which was a year 

 after I had elaborated the main lines of my own method, and applied 

 it with success to the study of young endosperms. The great merit 

 of Poirault's modification is that here for the h'rst time provisions are 

 made for preserving and hardening the tissue before taking sections. 

 New dyes are also used. With certain kinds of tissue this method 

 appears to have given excellent results. 



I may now introduce my own researches. In the course of obser- 

 vations on this particular branch of cytology, certain salient facts 

 come to the fore. In the first place one learns that material pre- 

 served in alcohol does not appear to be suitable for the investigation, 

 and consequently fresh tissue has been used. Secondly, that it 

 * Poirault, ■ Ann. Sei. Nat. (Bot.),' vol. 18, 1893. 

 f Kienitz-G-erloff, ' iiot. Zeit.,' 1891. 



I 2 



