652 The American Naturalist. [August, 
vidual sets in, concluding with an interchange of nuclear sub- 
stance. In each cell Maupas distinguishes between the (M) 
meganucleus (fig. 10, the macronucleus, nucleus, endoplast of 
authors), which presides over nutrition and growth and divides 
by constriction, and the (m) micronucleus (paranucleus, nucleo- 
lus, of authors), which presides over the preservation of the 
species. The latter contains chromatin; it is the seat of reju- 
venescence, the basis of heredity, it divides by mitosis, show- 
ing all the typical stages of karyokinesis excepting the loss of 
the cell membrane. 
The transformation in each of these copulating cells first 
‘affects the centres of hereditary substance, viz., the micronu- 
clei ; they divide three times; thus the micronuclear substance 
is reduced to one-fourth of its original bulk. It is contained 
in two surviving micronuclei (the others being absorbed or 
eliminated), one of which migrates into the adjoining cell; 
the other remains stationary. This migration is followed bya 
fusion of the migrant and stationary micronuclei ; this fusion 
effects a complete interchange of hereditary substance, after 
which the two Infusoria separate and enter upon a new life 
cycle. Meanwhile the meganucleus breaks up and is recon- 
stituted in each fertilized cell. 
Maupas gathers from these interesting phenomena addi- 
tional proof that the chromatin of all cells bears the inherited 
characteristics and that the cytoplasm and nucleoplasm, or 
achromatin, is the dynamic agent, because the micronuclei 
bearing the chromatin are the only structures which are per- 
manent and persistent, all the other structures—nucleoplasm, 
archoplasm, ete.—being replaced and renewed. The reduc- 
tion of the chromatin is purely quantitative, the eliminated 
and fertilizing micronuclei being exactly equivalent; after the 
chromatin has been quartered the cell becomes incapable of 
further activity until it is reinforced by chromatin from the — 
copulating cell. 
No Distinction Between the Sexes in Heredity —The three laws 
which underlie these phenomena are: 1. That fertilization — 
consists in the union of the hereditary substance of two indi- 
viduals. 2. That before union the hereditary substance in ) 
