midieæ, Zggnemaceoe, and in other Algæ, and which had already
been noticed and described by various writers under this significant
title. The facts, he observed, are thus recorded by
Mr. Thwaites ; and we have copied his figure to give greater
distinctness to his description ;—“ The process of conjugation
consists in the union of the endochrome of two approximated
fronds, this mixed endoclu-ome developing around itself a proper
membrane, and thus becoming converted into the sporangium.
In a very eai’ly stage of the process, the conjugated frastules
have their concave surfaces in nearly close apposition (Plate A.
fig. 2. I.) ; and it may be observed that from each of these
surfaces two protuberances arise, which meet two similar ones
in the opposite frustule : these protuberances indicate the futm-e
channels of communication by which the endochrome of the two
fi'ustules becomes united, as well as the spot where is subsequently
developed the double sporangium, or rather the two
sporangia.
“ The mixed endochrome occurs at first as two irregular masses
between the connected (conjugating) frustules, but these masses
shortly become covered with a smooth cylindrical membrane—•
the young sporangia (fig. 2. II.), which gradually increase in
length, retaining nearly a cylindrical form until they far exceed
in dimensions the parent frustules (fig. 2. I l l ) , and at length,
when mature, become, like them, transversely striated upon the
surface (fig. 2. IV.). Around the whole structure a considerable
quantity of mucus has, during this time, been developed, by
which the empty frustules are held attached to the sporangia.”
Nothing can be more accurate than this description, which
would apply to almost every instance of conjugation in the
Diatomaceæ.
A few circumstances omitted by Mr. Thwaites, which become
of significance when the process is observed in other genera,
will be noticed hereafter ; in the mean time it is interesting to
remark, how exactly conformable to the above account are the
phænomena attending the conjugation of other species of the
genus Epitliemia, viz. EpitJiemia Soreæ (Plate A. fig. 9), Epi-
themia gihba (Plate A. fig. 13), and Epitliemia ventricosa (Plate A.
fig. 14). .
In a postscript to the letter announcing the above discovery,
Mr. Thwaites states that he had subsequently detected the
process of conjugation and mature sporangia in two species of
Gomphonema and in Cocconema lanceolatum-, and in a paper
which appeared in the ‘ Annals ’ for November 1847, he details
and figures these and other cases of conjugation, in the discovery
of which I had the pleasure of aiding and co-operating
with my acute friend.
In a final communication which appeared in the ‘ Annals for
March 1848, Mr. Thwaites records two additional examples of
the conjugating process in Gydotella Kützinyiana and ScUzo-
nenia {Colletonemd) subcohoerens, and shows that the enlarged
frustules of the Melosireæ, which Kützing had conjecturally
regarded as reproductive bodies, were in fact the sporangial
product of conjugation, the original sporangia submitting to
self-division immediately on their formation, and thus forming a
chain of frustules larger than those from which they had themselves
originated (Plate XLIX. fig. 329).
No further observations on this singular process appear to
have been recorded by British naturalists until the appearance
of the first volume of the present work, in which I incidentally
mentioned the names of various species in. which I had detected
conjugation, deferring a detailed account of the phænomena
attending the process until the present opportunity of recording,
in consecutive order, all the examples I had noticed in the
British genera.
In the ‘Annals of Natural History’ for August 1855, an
interesting addition was made to the number of genera exhibiting
conjugation by Dr. J. W. Griffith, who had observed the process
in a Nmicnla (probably N. firma) occurring to him in a ditch
near Blackwall. The ‘ Micrographie Dictionary,’ in part edited
by the same author, in plate vi. fig. 5, March 1855, gives a representation
of Surirella hifrons, Kiitz., in a state of conjugation :
and lastly, in the ‘ Annals of Natural Plistory,’ January 1856,
we have an account of the phænomena attending the process in
Cocconeis Pedicuhis, Cymbella Pediculus, Kiitz., and Amphora
ovalis, as noticed by Mr. li. J. Carter of Bombay, all of which
appear to have occurred to him in vessels containing larger