QUEKETT MICROSCOPICAL CLUB. 267 



dot " and " black-dot " images. The markings consisted of 

 rectangular cells closed on both surfaces by a thin membrane, the 

 outer membrane being sunk in a very shallow pit below the surface 

 of the valve. The outer cell wall was found to be dotted. A 

 similar structure, discovered by Mr. E. M. Nelson, was described 

 as occurring in Pleurosigma formosum. Although formerly 

 accepting the view that diatom valves were perforated, Mr. 

 Brown is now of the opinion that there are no real holes in 

 them, and that the entry of fluids into the valves may admit 

 of another explanation. He thinks that a clue might be 

 found in Professor Dendy's discovery of some siliceous sponge 

 spicules that are capable of absorbing water, and suggested the 

 possibility of the cell walls of the valve being formed of some 

 such material. The growth of diatoms was then dealt with. 

 Mr. Brown does not accept the theory that growth of a diatom 

 does not take place. The theory that is generally held is 

 that the frustule keeps on forming new and smaller valves until 

 eventually conjugation between two frustules of the minimum 

 size takes place and an auxospore is formed, which is an individual 

 of the largest size that repeats the divisional process until the 

 minimum stage is reached and conjugation again takes place. 

 He finds evidence of growth difficult to obtain, although it 

 exists. For instance, a colony of a Cocconeis was once seen 

 consisting of diatoms of all sizes, and the lecturer felt con- 

 vinced that the colony was derived either from invisible spores or 

 from very young sporangial diatoms. Mr. Brown considers the 

 auxospore to be merely a sporangium from which young diatoms 

 are developed. Dr. Burton Brown made observations on En- 

 cyonema prostratum. He saw the diatoms conjugate and found 

 that after the auxos pores were developed a lot of young Encyo- 

 nemas appeared which grew larger, while the outside tests of the 

 auxospores remained where they were and gradually dissolved. 

 The so-called auxospore condition of Cymbella cistula was then 

 described and explained. The following is the method of reproduc • 

 tion as observed in Cymhella cistula by Mr. Brown and illustrated 

 by specimens under the microscope : A single adult, diatom sur- 

 rounds itself with a gelatinous sheath and a small mass of proto- 

 plasm is extruded from between the valves on the ventral side. 

 It is covered with a thin, structureless, siliceous membrane. An 

 organised baby diatom replaces the structureless mass, and from 



