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all are more or less invested exteriorly by a delicate film of 
sarcode, which (as long ago pointed out by Professor William- 
son, as being characteristic of the Foraminifera) is an effective 
agent in increasing exogenously the thickness and extension of 
the mineral portion, of whatever nature the latter may be. 
In short, as tersely put by Dr. Carpenter, whilst describing 
the perfectly developed cell-formation observable in the higher 
animals, the nucleus may be regarded as the seat of the celVs 
formative activity .* So in the sarcoblast (which is not a cell, 
and is therefore in no respect more entitled to the appellation of 
the term 44 cell ” than a globule of oil, albumen, or other viscid 
fluid, notwithstanding that it comprises one portion more con- 
solidated or specialized than the other), the central region is 44 the 
seat of its formative activity ” ; this very instructive fact having 
been, as already stated, completely verified, by what has been 
observed by me to take place, both in the marine and fresh- 
water Rhizopods. 
But the great importance of arriving at a definite and correct 
conception respecting this part of the Rhizopod structure will 
presently be made apparent on other and even stronger grounds. 
Meanwhile it must be mentioned that Mr. Mivart attempts, 
amongst other things, to determine 44 whether or not the re- 
semblance, insisted on by some writers, as existing between the 
Radiolaria and the Heliozoa is such as to render it desirable to 
unite them as two subdivisions of one larger group, itself dis- 
tinct from all the other groups of Protozoa.” Hertwig, he says, 
44 in his latest paper on the histology of the Radiolaria, leaves the 
question undecided whether the Heliozoa , Radiolaria , and Tha- 
lamophora , should constitute three distinct and co-equal groups ; 
or one great group should be made to comprise the Heliozoa 
and Radiolaria only, the other being set aside for the marine 
and freshwater Thalamophora ; Hertwig having in a prior paper 
denied the relationship of the Heliozoa to the Radiolaria ,” on 
the ground of the supposed cell-structure both of the external 
and internal alveoli, and 44 also of the Wasserhellen Blaschenrf 
as well as on other accounts.” Mr. Mivart, however, goes on to 
say that as Hertwig is now satisfied that 44 alveoli are but 
vacuoles, without membranous walls, and that the Wasser-hellen 
Bldschen are but nuclei , his reasoning falls to the ground ; ” — 
a conclusion in which I heartily coincide. 
The similarity of the Heliozoa and Radiolaria in external 
form and chemical composition, Mr. Mivart regards as undeni- 
* Carpenter on “ The Microscope,” p. 734. Fifth ed., 1875. 
t These are said to he “ rounded homogeneous particles of denser sarcode,. 
devoid of investing membrane ; and to be the 4 simple nuclei ’ ” of authors. 
(Memoir, p. 141.) 
