On ' Nitra^in ' and the Nature, &c., of Nodules of Plants. 63 



degree of specialisation of its outer portion, but no sign of any trans- 

 formation of the ameloblasts themselves into any intermediate form of 

 tissue has been observed. 



The sudden and entire transformation of the ameloblasts in the 

 Gadidse may perhaps be correlated with the very early and rapid for- 

 mation of the enamel, which is formed while there is yet- but little 

 dentine calcified. 



In mammals enamel formation is a very much slower and more 

 gradual process, and the dentine is always much further advanced 

 towards completion than is the enamel. 



A comparison of the various processes now known as occurring in 

 fish, in implacental mammals, and in placental mammals may, in the 

 author's opinion, be taken as finally disproving the idea, which is still 

 entertained by some, that enamel is to be regarded as a sort of secre- 

 tion shed out from the ends of the ameloblasts ; for, imperfect though 

 our knowledge remains in some respects, yet some form of conversion, 

 direct or indirect, of a pre-existent organic matrix is common to all, 

 though as in placental mammals it may be exceedingly small in amount, 

 and the erroneous idea alluded to has proceeded from the study 

 of the process exclusively in placental mammals, in whom its true 

 nature is most difficult to decipher. 



" Further Observations on ' Nitragin ' and on the Nature and 

 Functions of the JSTodules of Leguminous Plants." By Maria 

 Dawson, B.Sc. (Lond. and Wales), 1851 Exhibition Science 

 Eesearch Scholar. Communicated by Professor H. Marshall 

 Ward, F.RS. Keceived December 5, 1899, — Eead January 

 18, 1900. 



(Abstract.) 



In December, 1898, a paper by the author on " Nitragin, and the 

 Nodules of Leguminous Plants,"* was read to the Royal Society. 

 Since that time the work on this subject has been considerably 

 extended, and a brief summary of the additional results is given 

 below. 



Investigations have been made in the following directions : — 



A. Microscopic Observations. 



A comparative study of various points of interest in the anatomy of 

 nodules borne by several genera of different tribes of the order, with 

 special reference to the mode of growth of the nodule organisms 

 within the tissues of the host. 



* ' PhiL Trans.,' JS, voL 192, pp. 1—28. 



