GENUS ORBITOLITES : — REPRODUCTION ; VARIATIONS IN SIZE. 
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phenomenon of their development, instead of being the result of accident. — I do not 
wish to attach any weight to the interpretations I have here offered ; but I simply state 
the facts, and the explanations of them which have suggested themselves to my own 
mind ; merely adding, what I hope to present in more detail at a future opportunity, 
that bodies resembling the first or primordial cell, in which Foraminifera of all forms 
originate, are not unfrequently met-with in the chambers of many other species. 
IV. Variations. 
44. Variations in Size. — We have already seen that diversities both in the diameter 
and in the thickness of the disk, arise directly from the degree in which the animal 
substance (whereon the skeleton is modelled) has extended itself either horizontally 
or vertically, so as to multiply either the number of concentric rings, or the number 
of the superposed segments of which each ring consists. This, however, is not the 
only source of variation in size ; for a most extraordinary diversity presents itself in 
the dimensions of the individual components, by whose repetition the entire disk is 
made up. It is in the nucleus that I find this diversity most strongly marked, as 
will appear from a comparison of Plate VII. figs. 1 — 4, which exhibit parts of a grada- 
tional series of twelve, from the smallest to the largest forms I have met with, all of 
them accurately drawn, under the same magnifying power, from specimens in my 
possession*. The length of the entire nucleus of fig. 4 is about seven times that of 
the nucleus of fig. 1, and its breadth about four times as great ; the area of the former 
is therefore about twenty-eight times that of the latter; and as it is also several times 
as thick, the whole of the cavity, which was occupied in the living state by animal 
substance, could scarcely have been less than a hundred times as large in the one as 
in the other. (Compare also figs. 5, 6, 10, 12, 13 of Plate IV.) There is not by 
any means the same amount of difference between the dimensions of the ordinary 
cells which are formed by concentric extensions of the nucleus ; nevertheless, it will 
be seen by a glance at the figures just referred-to, that these also exhibit marked 
diversities in size, the largest cells being usually found to spring from the largest 
nuclei, and vice versd. Moreover, the individual cells of the very same disk are 
occasionally found to differ amongst each other, as widely as do the cells of fig. 1 
from those of fig. 4. 
45. Similar differences present themselves in the thickness of individual cells ; as is 
of course best seen in the simple type of Orbitolite, in which the augmentation of 
thickness is not produced by the vertical superposition of multiple segments. A 
remarkable example of this kind is presented in the comparison of figs. 4 and 5 of 
Plate V. ; these being, like the figures in Plate VII., drawn under the same magni- 
fying power. I possess a series of vertical sections of different individuals, in which 
the same gradual transition is seen from the thin to the thick, as I have just stated 
MDCCCLVI. 
* The entire series of figures is in the possession of the Royal Society. 
2 F 
