512 SUMMARY OF CURRENT RESEARCHES RELATING TO 



diatom is very rigid. In the young individual, while in a motile con- 

 dition, this is by no means the case, the cell-wall being extremely 

 flexible and elastic. The siliceous particles, which increase in number 

 with the age of the cell, are at first by no means fixed, and the entire 

 membrane is very flexible. This is readily seen when one individual 

 comes into contact with another. If a young motile cell is treated 

 with chlor-iodide of zinc, the entire cell, including the so-called 

 "lorica," takes a yellowish-brown tint. There is, in fact, no true 

 cell-wall ; the outermost siliceous layer is rich also in nitrogen, and 

 must be regarded simply as a denser layer of protoplasm. The con- 

 dition of the older cell is very diiferent, being then enveloped in a 

 brittle true cell-wall, destitute of nitrogen ; but in that condition it is 

 no longer motile. Very careful observation with a powerful oil- 

 immersion system, shows that the whole outer layer of the cell is 

 motile, the apices swaying backwards and forwards, and the layer dis- 

 playing constant irregular contractions, altogether resembling those 

 of the Flagellata. The statement is therefore correct, that movements 

 in the protoplasm are the cause of the motility ; not, however, in conse- 

 quence of rotation, but rather of contractions of the entire organism. 

 The general conclusions may be summed up as follows : — 



1. Frustulia, Navicula, Cymhella, and probably all motile diatoms, 

 have in the young condition no true cell- wall, but are, like the flagel- 

 late infusoria, naked protoplasmic cells, clothed by a delicate, con- 

 tractile, siliceous, extremely flexible layer, which only subsequently 

 becomes a true non-nitrogenous cell-wall. 



2. The motility is due to the contractile layer of protoplasm, and 

 is an extremely complicated phenomenon. 



3. There is no crevice in the mid-rib of the primary side. 



4. There is no regular rotation of the protoplasm which can serve 

 as a cause of the motility of the cell. 



5. The so-called '• box-hypothesis " is not confirmed (in Navicula 

 and Frustulia saxonica). 



6. Cell-division is preceded by an increase of the transverse dia- 

 meter of the secondary side of the cell in Frustulia and Navicula, 

 but there is no bursting of the outer cell-wall as in Melosira. This 

 is accompanied by a division of the nucleus, and a movement of the 

 two daughter-nuclei to the primary side. 



7. The cell divides by a longitudinal division-wall, which is at 

 first simple, but afterwards becomes double. 



8. After the separation of the sister-cells they increase at the ends 

 in a transverse direction, and thus form the gelatinous but siliceous 

 non-nitrogenous apices, united to one another by the longitudinal 

 ribs. 



Uses of the Study of Diatoms.* — Dr. M. Lanzi points out how 

 the study of diatoms is connected with medicine, geology, and 

 agriculture. 



As regards medicine, the well-known " edible earth " consists of 



* Lanzi, M., " Utilita dello Studio delle Diatomee," R. Medic. Accad. Roma, 

 Atti 1880, Seduta 28 Dec. 1879, pp. 1-13. 



