ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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wall and the wall of the nucleus, which takes the form of two necklace- 
like threads ; these contract and form two nucleolar segments. The 
wall and the tube of the nucleole then become absorbed. At the end of 
each segment a small slender thread appears ; they are distinguished 
from the nuclear segments by their greater resistance to chromic acid. 
The framework of the nucleus, with its twelve segments, retreats towards 
the equatorial plane of the nucleus, and forms the nuclear plate. In the 
process of division of the nuclear plate, a longitudinal fission takes 
place of the segments and of the resistant threads. The half-segments, 
twelve in number, coalesce, this process taking place about the same 
time as the division. The halves of the nuclear plate develop into 
nuclei, and the halves of the resistant threads into nucleolar threads. 
If the two nucleolar threads meet, they are surrounded by a common 
wall, and a nucleole is formed ; if they do not meet, each nucleolar 
thread invests itself wtth a wall, and two nucleoles are formed. 
When karyokinesis is not accompanied by the formation of segments, 
the nucleolar threads become shorter and thicker, and take the form 
of tubes with a thick wall. The wall of the nucleole then becomes 
absorbed ; the framework of the nucleus retreats to the equatorial plane, 
and forms the nuclear plate. When the nuclear plate divides, the two 
resistant threads which lie in the plane of division split longitudinally ; 
but the halves remain for a time united at their ends. The halves of 
the nuclear plate develop into nuclei, and the halves of the resistant 
threads into nucleolar threads. Usually the two nucleolar threads are 
surrounded by a common membrane, and form a nucleole ; less often 
two nucleoles are produced by the formation of a membrane around 
each thread. 
f Breaking-up of the Filaments of the Conjugate.* — Herr W. Be- 
necke explains the readiness to break up into their individual cells dis- 
played by many Conjugate, especially Mougeotia and Spirogyra, on a 
purely mechanical principle, quite independent of the action of oxygen. 
The cells in the filament are only held together by the cuticle ; the 
septa are split at an early period into two lamellae. The breaking up of 
the filament is the result either of a uniform increase in turgor of all 
the cells, or, more often, of the loss of turgor in particular cells. Dead 
or injured cells are in this way thrown off ; and probably the same pro- 
cess facilitates the separation of conjugated cells from the rest of the 
filament. 
Diatom Structure.^ — Mr. E. M. Nelson describes a variation in 
Coscinodiscus Asteromplialus, in which, instead of the well-known pattern 
consisting of a ring of larger areolations surrounding the finely per- 
forated membrane which covers a large polygonal cell, we have a 
circular ring of brackets projecting inwards to strengthen this delicate 
membrane or cover. 
He also records the interesting discovery, in a specimen of the same 
diatom of the common form, of a tertiary structure which he believes to 
be a stage in the evolution of the central perforated membrane. He 
adduces reasons for regarding the following as an enumeration of the 
0 2 
* Jahrb. f. wiss. Bot., xxxii. (1898) pp. 453-77 (1 fig.)d 
f Journ. Quekett Micr. Club, vii. (1898) pp. 81-7 (1 pi.). 
