56 THE AMERICAN NATURALIST. [VoL. XXXIV. 
diminished volume and so are nearer together; they do not travel 
to such regions. 
Though the reader may not be convinced of the necessity of this 
explanation, it is certainly most interesting to learn that bubbles of 
air squeezed through mashed egg masses, and even through artificial 
mixtures of soot and emulsions, do leave pigmented tracks behind, 
similar to the sperm track. Some physical explanation of the pig- 
mented sperm track seems forced upon us. 
The author then takes up the concentration of pigment upon part 
of the surface of the frog’s egg; the aggregation of pigment about 
the nucleus in cleavage stages of the frog’s egg; the remarkable 
rearrangements of pigment in isolated cells of frog’s eggs when 
“cytotropic’? movements bring them together again; the arrange- 
ments of pigment in blastulz of triton after injury by pressure; and 
the normal arrangement of pigment in certain cells of the triton in 
the stages of gastrulation and of formation of neural ridges. 
In all cases he applies the same formula: pigment collects in 
areas of increased pressure and of condensation. 
When cells are in active chemical interchange there will be greater 
adhesion of their applied surfaces and less cohesion of the part of 
each turned towards its neighbor. The parts of the cells most 
removed from such interchange will be those of relatively high ten- 
sion, and in these the pigment, if present at all, may be concentrated 
by adhering to the shrinking lamella. Pigment will be on the 
denser and chemically inactive sides of cells. 
In early stages of development pigment does not indicate active 
chemical changes where it is found, nor is it of any direct use; it 
remains inert and is concentrated in areas of condensation or may 
even be expelled from active areas. 
Secondarily, natural selection may have emphasized its occurrence 
in some places where it happened to be of use. 
Aside from the main issue, the author's evidence that invagination 
and evagination are due to actions of individual cells seems specially 
interesting. E A A. 
Biological Lectures. — The substantial character of the work done 
at the Marine Biological Laboratory at Woods Holl is evinced by 
the recently published volume of Biological Lectures for 1898.' The 
sixteen lectures thus brought together are by well-known authorities, 
and touch on one side or another almost all the important biological 
1 Biological Lectures, from the Marine Biological Laboratory, Woods Holl, 
Mass., 1898. 343 pp. Ginn & Company, Boston, 1899. 
