398 DAVID H. TENNENT 
these lagging chromosomes may be seen by their size and form 
to be Toxopneustes chromosomes. 
Fig. 11 A, B and C, chromosome formula 33. Long rods and 
V’s in anaphase plates; some Toxopneustes chromosomes lagging 
between the plates. 
Fig. 12 A, B and C, chromosome formula 3?. Some of the 
chromosomes in 12 C are probably fragments of those in lower 12 B 
Toxopneustes V’s not present. Apparently there has been an 
elimination of Arbacia chromosomes. 
Fig. 13 A and B, chromosome formula 33. One Toxopneustes 
V and the long rod present. Lagging chromosomes probably of 
Toxopneustes. There seems to have been an elimination of Arba- 
cia chromosomes. 
Fig. 14 A and B, chromosome formula 33. Long Toxopnuestes 
rod absent. Some of both Arbacia and Toxopneustes chromo- 
somes have been eliminated. 
Fig. 15 A and B, chromosome formula 37. Toxopneustes V’s 
not present. Full number of Arbacia chromosomes does not seem 
seem to be present. 
Fig. 16 A, B and C, chromosome formula 3}. Toxopneustes 
V’s not present. There has been an elimination of both Toxo- 
pnuestes and Arbacia chromosomes. © 
Fig. 17 A, B and C, chromosome formula $2. Pronounced lag- 
ging of Toxopneustes chromosomes between daughter plates. 
THE TOXOPNEUSTES 2 X ARBACIA co’ CROSS 
Passing now to the reciprocal cross, figs. 18-21, we notice at 
once a difference in the appearance of the figures, the number of 
chromosomes present in each plate being less than in the plates 
of the reciprocal cross. 
Fig. 18 A and B, chromosome formula $$. Both of the Toxo- 
pneustes V’s and the long rod are present. Most of the Arbacia 
chromosomes have failed to enter the division figure. 
Fig. 19 A and B, chromosome formula 7}. Both of the Toxo- 
pneustes V’s and the long rod are present. Nearly all of the 
Arbacia chromosomes have been rejected. 
