O’Mrara—Report on the Irish Diatomacee. 239 
plasm-sac and the structure which is called the middle-mass. In case 
the normal condition of the cell-contents be disturbed by fracture of 
the silicious epiderm, the endochrome plates go together, and never 
commingle with the material of the plasm-sac. If the colouring 
matter be discharged by alcohol, the demarcation of the endochrome 
plate from the rest of the plasm can be readily distinguished. 
Within the folds of the endochrome plates is found in some a 
collection of plasm, which Pfitzer calls the ‘‘ middle plasm-mass,”’ 
‘described by Ehrenberg as resembling ‘“‘ the embyro in an egg;”’ in 
_ the Navicule it forms generally an irregular quadrangle. Vacuoles and 
oil globules occur imbedded in this middle plasm-mass, and appear 
distinctly in consequence of their strong refractive power. In the 
middle of this plasm-mass a central vesicle 1s observable in some 
genera, but is not equally distinct in all species. And although in some 
cases it cannot be discovered, even with the most skilful management, 
Pfitzer considers that nevertheless the statement of Liiders may be 
‘correct, that no Diatomaceous cell is destitute of such a vesicle, because 
although in many cases no such structure can be detected by ordinary 
means, it becomes apparent by the application of re-agents, the most 
-effective for the purpose being dilute hydrochloric acid. 
Besides the parts already specified, there have been observed in 
some of the Diatomaces a water-like fluid substance, and oil-globules, 
varying in size. ‘These latter occur swimming freely in the cell, but 
in greater number upon the inner surface of the plasm-sac. In con- 
sequence of their strong refractive power they strike the eye at once, 
and are changed into a black colour by the use of osmic acid. As 
they readily combine, they have no investing pellicle. It is thought 
that in proportion as the oil-globules abound, the cells have suffered 
from the want of pure water, and that the appearance of the larger 
-oil-globules is a sign that the cell has attained its full maturity, 
and that its resources have been exhausted. The oil-globules afford 
a means of answering the question whether the cell contents are of a 
watery or of a gelatinous consistency. In favour of the former view, 
Pfitzer refers to the fact that very weak acid produces an immediate 
shrinking of the plasm-sac, and also to his observation that the oil- 
globules can be moved about with facility, which could not occur 
if the surrounding matter were of a gelatinous thickness. And this 
opinion of Pfitzer has been corroborated by Focke, who discovered 
that the oil-globules, in consequence of their light specific gravity, 
accumulate on the upper surface of the cell, and change their position 
in case the frustule is turned upside down. 
The Reproduction of the Diatomacee 
Is a subject of deep interest, requiring some explanatory remarks. 
The ordinary mode of increase is by self-division, as it has been 
termed. The cell-contents within the enclosure of the silicious 
epiderm separate into two distinct masses. As these develop they 
