666 The American Naturalist. [August, 
forces in the:ovum which prevent multiple fertilization or 
bastard fertilization (i. ¢., by spermatozoa of other varieties), 
but these forces are neutralized where the life-energy of the 
cell is diminished by reagents or by extremes of temperature. 
For example, in the normal state the entrance of a single 
spermatozoon produces a reaction in the ovum-wall preventing 
the entrance of other spermatozoa, but when the ovum is weak- 
ened by chloroform solution two or more spermatozoa enter 
before the reaction appears; in fact the degree of polyspermy 
is directly proportional to the intensity of the chemical, ther- 
mic, or mechanical disturbance of the ovum. Double fertili- 
zation or over-fertilization has not in a single case resulted in 
the production of twins, so that Fol’s supposition is negatived, 
although other forms may behave differently. The cell-func- 
tion may be arrested at any stage by thermic influences ; thus 
two pronuclei, paternal and maternal, about to unite can be 
held apart by lowering the temperature. Polyspermy also 
results from a lower temperature. It is noteworthy that the 
conditions of bastard fertilization and polyspermy are differ- 
ent; chloroform produces the latter but not the former. 
Kupffer has, I believe, succeeded in producing twins, or rather 
two-headed monsters, by abnormal fertilization in fishes. 
These researches, although made with a different object, 
re-establish the older views as to the interdependence of nuclear 
and extra nuclear activities, and show that no sharp line of 
demarcation of function can be drawn between the nucleus as 
a center of reproduction and heredity, and the cytoplasm as 
the seat of tissue-building and nutrition. In Boveri’s discov- 
ery of the archoplasmic centres, or centrosomes, we find posi- 
tive ground for this broader view. It is connected with the 
cell phenomena of heredity in the following manner: : 
While the union of the nuclei in fertilization is the most — 
obvious feature, this union is dependent upon the archoplasm, 
which rearranges the nuclear elements. If the spermatozoon 
contains no archoplasm, this power cannot come from the 
paternal side; but Boveri shows that this is probably not the 
case and that the spermatozoon brings its centrosome with it, 
thus entering the ovum with both the paternal chromatin sub- — 
