1098 The American Naturalist. [December, 
These departures from the normal might be due to the direct effect 
of the pressure acting upon the dividing nuclei or else to the secondary 
effects brought about through the change of form the egg suffers under 
the pressure. 
While Pflüger was inclined to regard the pressure as acting directly, 
as determining the direction in which the nucleus could most easily 
elongate, the author thinks that the change of form of the protoplasm 
is the determining cause of the new arrangement of the cleavage 
planes. He thinks that these phenomena may be brought under the 
rule formulated by O. Hertwig, that in dividing the nucleus tends to 
get into the center of its field of action, the surrounding protoplasmic 
mass, and places itself so that its poles are toward the largest masses of 
protoplasm. 
It is thus not the pressure which directly alters the position of the 
nuclear spindle but the forced change of form of the protoplasm of the 
cell which necessitates an adjusted position of nucleus and hence a sub- 
sequent change in the direction of the cleavage. 
If the author succeeds in making out in detail this mass effect of 
these distorted cells we would seem to have additional reason for 
regarding the nucleus as of less value than the protoplasm in the deter- 
mination of form. 
Embryology of Chiton.—Dr. M. M. Metcalf studied the embry- 
ology of Chiton marmoratus and C. squamosus at Jamaica, where the 
Marine Laboratory of the Johns Hopkins University was located in 
1891. An account of the breeding habits, methods of studying the 
small opaque eggs (use of hardening liquids, hypochlorite of soda to 
remove the chorion and yolk, etc.) and a detailed, illustrated descrip- 
tion of the cleavage and gastrulation is given in the last number of the 
Studies of the Biological Laboratory, Baltimore, October, 1893. 
_ The eggs were obtained from specimens kept in aquaria; both eggs 
and sperm are discharged for a period of two hours or more after a 
time of active, sexual excitement. The males and females, however; 
do not approach one another but seem to give off the sexual cells 
under the stimulus of some unknown influence. 
The cleavage is described and figured in great detail chiefly from 
surface views of living eggs, and from reconstruction of sections up to 
the forty cell stage. The gastrulation also is given in detail and pre- 
sents interesting features with reference to the slit like form of the 
blastopore in Peripatus. Later stages are reserved for subsequent 
work, In general the results here given. confirm the work of Kowa- 
levsky. 
