REVIEWS. 
Biologische Studien. Von Dr. Ernst Harcxen, Erstes 
Heft. Studien ueber Moneren und andere Protisten. 
(See Plate V.) 
Tue volume which Dr. Haeckel has just published con- 
tains reprints of several papers already put out by him in the 
‘Jenaische Zeitschrift,’ together with two additional notices 
of new forms of Monera which we have not previously seen. 
The monograph of the Monera is already well known to our 
readers, since it was reproduced in full in this Journal during 
the year 1869. Succeeding this in the present volume is a 
series of papers entitled “Contributions to the Plastid 
Theory.” Inthe first of these Haeckel points out what he 
means by the ‘ Plastid theory,’ which he discusses in connec-. 
tion with the generally received ‘ Cell theory.” Just as atone 
time the cell was conceived to be the simplest living form, as 
seen in the ovum and unicellular organisms, and just as it 
was conceived that organisms are built up by aggregations of 
these simplest morphological units, so must we now admit 
the existence of still simpler units—the simplest conceiy- 
able—mere bits of protoplasm, undifferentiated, without 
nucleus, living freely as Monera, and possibly also becoming 
aggregated also to form tissue. That such units should 
exist is what we were gradually led to expect by the re- 
searches of Max Schultze and others, resulting in the aban- 
doning of the cell-wall, and the rise of the all-important 
protoplasm-theory. It is Haeckel who has discovered them. 
He calls these simplest units Cytods, and classes Cytods and 
Cells together under the head Plastids. The cytod being the 
simplest possible possible form of life, it is this form under 
which life first appeared, and it is this which we should look to 
see formed by so-called spontaneous generation. In the course 
of development the cytod has given rise to the cell by in- 
ternal differentiation of a nucleus. Since the development 
of the individual (Ontogeny) is a more or less complete 
epitome of the development of the species (Phylogeny), we 
should expect such Plastids as are cells to pass through the 
cytod condition in their life-cycle, and we find that they 
