84 The American Naturalist. [January,. 
cell. When several cells are together the arrangement of the pigment 
appears to be in some way determined with reference to the arrange- 
ment of the cells, being at like poles or in zones, ete., according to the 
way the cells are combined. 
The bearing of these facts upon normal development becomes evident 
when we recall that not only are rearrangements of cells of importance 
in the normal processes of cleavage in many eggs, but that they also 
seem to play an important part in the formation of the later embryo in 
the case of the eggs of Ascaris as emphasized by O. zur Strassen.’ 
Such movements of cells may then he looked for as a not unimpor- 
tant factor in the production of the characteristic shapes and organs of 
embryos. 
The explanation of the nature of these movements is by no means 
ready. The author recognizes that simple surface tension of a homoge- 
neous material will not account for all the phenomena, but he is in- 
clined to think that surface tension may be a sufficient cause provided 
there were a change in its character at different parts of the same cell 
and at different times in the same area owing to some change in the 
nature of the material of the cell. 
A Mechanical Explanation of Cell Division.—As the phe- 
nomena of cell division form so large a part of the visible changes the 
embryologist studies, he will eagerly welcome any clue to their better 
understanding. Especially when we are offered an explanation of ; 
the complex changes of indirect or mitotic cell division, which figure 
so strikingly in the important early changes the egg undergoes. If 
weary of the idea of muscle-like contractility of fibres or the mysterious 
movements of chromosomes under chemical influences he will turn with 
relief to the mechanical views presented by Dr. Ludwig Rhumbler.* 
The keynote of this honest attempt lies in the assumption that the 
observed physical changes of cell division may be due to purely physical 
causes, whatever the complexity and differences of the unknown chem- 
ical factors lying back of these physical changes. 
The author first assumes that protoplasm is a viscous fluid, next that 
it has essentially the structure claimed by Bütschli, that is he regards 
protoplasm as a froth or foam of more liquid drops or alveoli surrounded 
by less liquid surfaces or lamellæ—in which may be fibres of soft gran- 
ules arranged in rows, 
In such a foam radiating lines may appear from the arrangements 
of the vesicles or alveoli and the author assumes that the radiations in 
"See American Naturalist, Dec. 1896, p. 1059. 
* Archiv f. Ent. der Organismen, an Taly 21, 1896, pps. 527-618. 
