244 REV. E. o'meara. 



It is necessary to premise that the views here expressed 

 have special reference to the Naviculacete, but are, with some 

 modifications, more or less appropriate to the other genera. 



Attention is first directed to the plasm-sac (Plasma-sch- 

 lauch), consisting of a fine colourless plasm forming a closed 

 sac of the shape of the cell, and which in the NaviculacetP, 

 as in all the other Bacillariacete , envelopes the cell-contents. 

 It is often very difiicult for the observer to make himself 

 certain of the existence of this sac, because its refractive 

 power diflTers but slightly from that of water, but the struc- 

 tiire becomes apparent on the application of dilute hydro- 

 chloric acid. The effect of this reagent is to produce an 

 immediate contraction of the sac, which at first, as it recedes 

 from the cell-wall, preserves the form of the cell, and still 

 maintains connection with it by a few pellucid threads, but 

 after some time becomes contracted into a round mass. This 

 result is brought out most effectively by the use of osmic 

 acid. Pfitzer informs us that Bacillarite, which have been 

 treated with a solution of the latter acid, at the strength of 

 one per cent., may be satisfactorily preserved in a mixture of 

 glycerine, alcohol, and water. This mixture should be 

 applied to the edge of the covering-glass, and, after being 

 allowed to evaporate, applied again. He further informs us, 

 that iodine gives a bright yellow colour to the plasm-sac. 



The next detail of internal structure to which attention is 

 called, in the work under review, is what the author describes 

 as a larger accumulation of plasma occurring in all the 

 Navicula, and which lies transversely in the middle of the 

 cell. This collection of plasma had been pointed out by 

 Nitzsch. Kiitzing noticed it, and gave it the designation of 

 transverse band (Querbinde). Ehrenberg describes it as 

 being like " the embryo in an egg." Other writers also 

 called attention to it, and finally Schiilze gave the most 

 accurate description of it in the case of Pleurosigma. This 

 middle plasma-mass is discoverable on the side view in both 

 valves, and forms generally but not universally a sort of 

 irregular quadrangle. Vacuoles and oil-globules occur im- 

 bedded in the plasm, and appear distinctly in consequence 

 of their strong refractive power. Here also, in the case of 

 the larger forms, Pinnulariee, for instance, have been observed 

 with more or less distinctness short, dark interrupted lines 

 which, in parallel pairs, pervade this central mass of plasma, 

 but are most abundant from the centre to the cell-wall. 

 These lines exhibit the same aspect in all positions of the 

 frustule, and therefore are supposed by the author to be 

 cylindrical threads of a thicker consistency than the re- 



