186 FOOT-NOTES TO EVOLUTION. 
operation, however, effectively precluded any success, 
and the eggs did not develop. 
Boveri selected for his experiments the eggs of two 
different species of sea urchins, Echinus microtuberculatus 
and Stherechinus granularis, both found in abundance at 
Naples. He found that, if the minute eggs of either of 
these were shaken vigorously in a test tube, in a small 
amount of sea-water, for a few minutes, they would 
break up into variously sized fragments, some of which 
contained nuclei while others did not.. These fragments 
could be fertilized, and development would proceed reg- 
ularly, the dwarf larve resulting resembling in all par- 
ticulars the normal larve, save for a small percentage 
of deformation caused by the shaking. This develop- 
ment took place in the xon-nucleate fragments as well as 
in those which contained nuclei, the spermatozoa pene- 
trating into both with equal readiness. The larval 
forms of the two genera Lchinus and Shpherechinus are 
so sharply defined that they can be recognised at the 
end of the second or third day’s development with un- 
failing accuracy. Fig. 13, C and J, illustrate in front 
and side view a normal larva of Spherechinus, Fig. 13, 
A and B, the same of Echinus. The general contour 
and shape of the larva, and more especially the widely 
differing calcareous skeletons of the two forms, render 
their ready distinction a very easy matter, and a closer 
analysis will reveal a large number of minor points of 
difference. 
After determining these facts, Boveri’s next step was 
to cross-fertilize the eggs of Spherechinus with sperm 
from Echinus. In asmall percentage of cases this suc- 
ceeded, and the resulting larve were uniformly of a 
type as shown in Fig. 14, 4 and JZ, in front and side 
view—a form which stands midway between the nor- 
mal larve of the parents, and combines the character- 
