412 SUMMARY OF CURRENT RESEARCHES RELATING TO 



discus in normal sections shows that their growth is at the free edge 

 (Wallich), and not by intussusception or the addition of a third in- 

 ternal connective (Cox). The newly formed valves consist of a single 

 very thin and perforated layer of silica ; their development is centri- 

 fugal (O. Miiller). In some preparations the internal surface of the 

 valves of Trinacria is covered with a black and opaque membrane, 

 showing the same perforations. It is j)robably the lowest layer of the 

 cellular envelope, slightly silicified or even entirely organic, reduced 

 to the condition of carbon by a slow combustion of its cellulose. 

 The existence of this layer is admitted by many authors (Dippel). 

 Chemical analysis proves, moreover, that this blackish substance is 

 charcoal. Inside the frustules it presents the appearance of rounded 

 spheroids ; sometimes it comidetely fills them. In a normal section 

 of Coscinodiscus it is established that it stops up the perforations of the 

 lower siliceous layer of the valves, and pushes prolongations into the 

 hexagonal cavities of the alveolar layer. 



Amongst the mineralized diatoms found in the London clay, and 

 in which the silica has been replaced molecule by molecule by iron 

 pyrites, there are found some also showing perforations (Kitton). 

 Certain Coscinodisci, closely resembling the species from the rock at 

 Fiir, often show, after cleavage, the lower layer and its perforations. 

 The sections of this clay which the authors are about to undertake 

 will render this demonstration still more clear. 



Motion of Diatoms.* — The following statement by Mr. C. Onder- 

 donk must for the present be received with some caution : — 



" The motion of diatoms is caused by what I will call the motile 

 pallium — a gelatinous, invisible envelope, that entirely envelopes the 

 diatom in the case of the strong-moving Naviculce, but only partially 

 in the case of other forms of weaker motion. This is no mere theory, 

 though I had worked out the theory long before I succeeded in making 

 the pallium plainly visible, and turned my whole attention to stain- 

 ing the motile matter long before I saw a trace of it. I have at length 

 succeeded in staining, hardening, and detaching the pallium, and I 

 now have many of them mounted, also many diatoms, in all stages of 

 disrobement, if I may use the term. The pallium is folded to the 

 diatom in many minute corrugations. Under the action of the 

 reagent, the minute corrugations slowly begin to expand out from 

 the diatom like pseudopodia ; longer and longer they grow, but soon 

 we see they are not pseudopodia, for they straighten out into a 

 membrane. The unfolding mantle splits along the midrib, and, in 

 some cases, leaves the flinty shell. This is not the membrane spoken 

 of as investing the diatoms by many investigators, as this is at once 

 destroyed by alcohol or acids. TJie same reagent reveals a similar 

 envelope on the OscillarioB, and I believe the motion to be the same 

 in the two forms of protophytcs. In fact, if a diatom was a long 

 elastic rod, it would merely vibrate, for I have observed that the 

 motion is generally in opposite directions on the two valves ; but the 

 diatom can only move by creeping along a surface, hence its motion 



* Amcr. Mon. Micr. Joiiru., iv. (1883) pp. Gl-2. 



