1 86 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 Sph(Erechinus granulans, 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 larvae resulting resembling in all par- 

 ticulars the normal larvae, save for a small percentage 

 of deformation caused by the shaking. This develop- 

 ment took place in the non-nuc/eate 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 Echinus and Shphcer echinus 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 Z>, illustrate in front 

 and side view a normal larva of Sphcer echinus, Fig. 13, 

 A and B, the same of Echi?tus. 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 Sphcerechinus with sperm 

 from Echifius. In a small percentage of cases this suc- 

 ceeded, and the resulting larvas were uniformly of a 

 type as shown in Fig. 14, A and B, in front and side 

 view — a form which stands midway between the nor- 

 mal larvae of the parents, and combines the character- 



