THE CHROMOSOMES 147 
species of sea urchins. The best analyzed cases are 
those that Baltzer has worked out. Crosses were 
made between four species of sea urchins; one such 
cross will serve as an example (Fig. 39). The eggs 
of Spherechinus were fertilized by the sperm of 
Strongylocentrotus. The division of the chromo- 
somes proceeded in normal manner. The pluteus 
a a 
3 Ye 
an we 
M 2 WA ats 
3 Ba 
Fic. 39.—1 and 1a, chromosomes in the first normal cleavage spindle of 
Spherechinus; 2, equatorial plate of two-cell stage of same; 3 and 3a, 
spindles of two-cell stage of egg of hybrid of Spherechinus by Strongy- 
locentrotus; 4and 4a, same, equatorial plates; 5and 5a, hybrid of Strongy- 
locentrotus by Spherechinus cleavage spindle i in telophase; 6, next stage 
of last; 7, same, two-cell stage; 8, same, later; 9, same, four-cell stage; 
10, same, "equatorial plate in two-cell stage (22 chromosomes); 11, same, 
from later stage, 24 chromosomes. (After Baltzer.) 
that developed was intermediate in character; or at 
least showed peculiarities both of the maternal and 
of the paternal types. The reciprocal cross was made 
by fertilizing the eggs of Strongylocentrotus with the 
sperm of Spherechinus. At the first cleavage of the 
egg some of the chromosomes divide normally, while 
other chromosomes remain inactive and finally be- 
