196 PROBLEMS OF FERTILIZATION 



sperm. A slight cytolysis of the egg is presumably 

 thus induced. The development was highly abnormal 

 in all cases. 



4. Sea urchin eggs have also been crossed with 

 sperm of different phyla. Kupelwieser (1909 and 191 2) 

 has made a special study of this problem. He investi- 

 gated the effect of the sperm of fourteen genera of 

 mollusks and annelids on sea urchin eggs and obtained 

 positive but usually scanty results in five cases, the 

 others being negative. A high concentration of sperm 

 and long exposure of the eggs was necessary. In all 

 these cases membrane formation of the egg might also 

 be induced by dead sperm or blood of the species. 

 Strongylocentrotus ? X Mytilus $ gave the best results. 

 The success of the fertilization seemed to depend on 

 extract present with the sperm, which so affected the 

 surface of the egg that one or more spermatozoa 

 could enter. But if membrane formation occurred too 

 rapidly, as a result of the sperm extract action, the 

 sperm did not enter, and the eggs died. Once within 

 the egg, if the condition was monospermic, events 

 moved normally to a certain stage; an aster formed in 

 association with the sperm nucleus; it then formed an 

 amphiaster while the germ nuclei united. The male 

 nucleus did not, however, form normal chromosomes 

 and was eliminated; but the female nucleus formed its 

 chromosomes, which divided in the usual way, and all 

 nuclei were henceforward haploid and purely maternal. 

 In a very small percentage of cases development might 

 proceed to the pluteus stage, which was usually defec- 

 tive. It was purely maternal as far as it went. In the 

 very usual event of dispermy or polyspermy the phe- 



