5^9 
Constituents of the Cell. 
the real nature of the ‘ sickle-stage,’ it seems probable that it 
will prove to be a nucleolus, and not chromatic substance, as 
I formerly thought, but displaced and flattened by the unequal 
penetration of the fixing fluid, as before indicated. Strasburger 
appears to share this belief. The evidence we now have is 
quite too slight to justify the attachment of any importance 
to this particular form. 
It is not yet nine years since the fundamental importance of 
the structures variously known as attraction-spheres, directive- 
spheres, and centrospheres, was first suggested, almost simul- 
taneously, by van Beneden and Neyt and by Boveri ; and it is 
four years less since they were described for plant-cells by 
Guignard ( J 91). Since then they have been observed by 
several investigators in various vegetable tissues ; but, on the 
other hand, it is certain that not all the structures which have 
been so regarded have really been centrospheres or centro- 
somes. The small size and comparative inconspicuousness of 
these bodies, which caused them to be so long overlooked in 
animal cells, are still more pronounced in case of plants, and 
make their study one of extreme difficulty. In the best and 
clearest vegetable preparations yet obtainable, their demon- 
stration leaves very much to be wished for, in comparison with 
successful slides from animal tissue. 
There are now generally recognized two more or less 
distinctly differentiated parts of these bodies, and the limi- 
tations of the names applied to them by Strasburger (’92), 
though not all first used by him, has been largely adopted by 
both botanical and zoological writers. A tiny central body, 
the cenirosome , is usually distinguishable from the surrounding 
globular, hyaline mass, the astro sphere ; these portions together 
constitute the whole structure, the centrosphere. In plants the 
centrospheres have been seen only in the cytoplasm, usually 
in intimate relations with the nucleus ; but it is only under 
most favourable circumstances that they can be recognized. 
The centrosome is little or no larger than the larger micro- 
somes of the cytoplasm, and can only be recognized when 
it is surrounded by an astrosphere of sufficient breadth to 
