110 
integrate and pass into the reticulum although their outlines 
can be dimly made out even after the reticulum is well formed 
and the nucleoli have reappeared (Fig. 16). 
5. Lhe centrosome. 
It is extremely difficult to stain, and correspondingly hard to 
find the centrosomes in this material; even at the spindle-poles 
its identification is not easy. I was able, however, to make it out 
in two different stages (Figs. 11,12 and 13). One of these was 
in the mitosis of the primary sporocyte, the other in that of the 
secondary sporocyte. In the first of these the centrosome at the 
spindle-poles was double (Figs. 11 and 12), in the second it was 
single (Fig. 13). 
II. TETRAD-FORMATION. 
In cases where reduction in the Weismann sense is actually 
known to take place, there have been wide variations in the ac- 
counts of the process. It was first described by a former pupil 
of Weismann’s, Ishikawa, who did not find tetrads and who held 
that reduction in the copepod Diaptomus is accomplished by the 
separation of entire chromosomes. This result is entirely contra- 
dictory to the more recent results obtained by the subsequent 
study of Diaptomus and other copepods. 
Vom Rath, apparently the first to correctly interpret the for- 
mation of tetrads, gave a different account of reduction in the in- 
sect Gryllotalpa. He found that the spireme is double before it 
breaks up and that there are half as many of the double segments 
as there are chromosomes in the somatic cells. The halves of the 
double segment separate, except at the ends, and a ring is formed. 
Later the tetrads arise by concentration of the chromatin at four 
points of the ring, a method by which the four parts of the tetrad 
originate by longitudinal division represented by the original divi- 
sion of the spireme, and by a transverse division. 
Hicker gave still another description of tetrad-formation in 
Cyclops strenuus. His details have been denied by Rickert, who, 
however, accepted the general results and agreed with him that 
the tetrad is formed as in Gryllotalpa by a longitudinal and a 
transverse division of the original spireme segments. Riickert 
(1893 and 1894) has found two modes of tetrad formation, each 
