116 NATURAL SCIENCE [August 
Three modes of nuclear reduction have been described :— 
Case J.—In Elasmobranchs, some Amphibia, and Mammals; in 
Flowering Plants, some Archegoniates, and Fucaceae. The nuclear 
network resolves itself at once into half the previous number of 
segments; certain modifications occur which do not alter the 
principle of the matter. Here there is clearly no true ‘ reduction, 
any more than if a man received ten shillings from his father and 
left five florins to each of his two sons. 
Case II.—In Sagitta and Ascaris, in some Gasteropods, and 
in Liverworts, a modification of this process has been made out, 
which was first noted by Boveri and Hertwig in 1890; while 
its relation to Case I. was dwelt on by me in 1891. Here the 
reduced number of segments appears in a certain cell; but after 
the first splitting for the coming division a second splitting of each 
occurs, so that at the first division of the nucleus each daughter 
nucleus receives the reduced number of segments ready split for the 
second division; and at the second division the again new daughter- 
nuclei receive each its own set. This is merely a displacement in 
time of the two successive splittings for two successive divisions of 
the nucleus. Similarly, often when a cell is going to undergo two 
or more successive divisions, the nucleus undergoes the consecutive 
divisions before the cytoplasm divides once. The reduction is 
essentially of the same character as in Case I. 
Case III.—The nucleus about to divide reveals a number of 
tetrads or groups each of four segments; the number of groups is 
only half the previous number of segments, and consequently the 
total number of segments is twice the original number: the nucleus 
divides twice, and at each division the several groups are halved 
between the resulting nuclei; hence it is clear that at the second 
division each nucleus contains one segment from each group—ze., 
a number equal to the number of groups, and half the original 
number of segments in previous cells of the cycle. 
Concerning the details of Case III. there are many conflicting 
observations, many inconsistent explanations, many vain hypo- 
theses based on the assumption that this is the typical mode of 
nuclear reduction to which all others are to be forced to conform 
by some Procrustean process. But we shall not go into these; the 
explanations that cover Cases I. and II. will cover Case III. also, 
even if in the last there be something additional left over; but the 
explanation of this something can well wait until the facts them- 
selves are better made out. 
Where does nuclear reduction occur? In Metazoa, usually at 
the first of the two cell-divisions that give rise to a brood of four 
spermatozoa, or to the oosphere and the three polar bodies (abortive 
oospheres) respectively: that is to say, at the inception of the forma- 
