3 '2 2 MESSRS. C. SHEARER, W. DE MORGAN, AND H. M. FUCHS 



We would point out, however, that although abnormal mitoses are of exceedingly 

 common occurrence (invariable in some cases) in the first segmentation stages of 

 Echinus hybrids, yet the number of abnormal larvse found in the cultures is much 

 too small to support Kupelwieser's hypothesis. For example, in the E. acutus and 

 E. esculentiis crosses it quite frequently happens that two daughter chromosomes 

 pass to the same pole, yet the percentage of normal larvae in such cultures is just as 

 high as in cultures of the pure species. 



In this connection, some experiments which DE Vries made in crossing (Enothera 

 throw considerable light. Their cytology has been recently investigated by 

 GoLDSCtiMiDT. De Vries (99) found, on crossing GH. muricata ^ X CE. biennis $ , 

 that he obtained only hybrids that resembled GE". hiemiis ; on crossing CE. biennis ? 

 X (E. muricata $ , only those that resembled (E. muricata. These hybrids bred 

 true through four successive generations. When he crossed these with one another 

 he found that CE. biennis ^ X CE. muricata $ [Fj (B g ) X Fj (M $ )] gave 

 CE. muricata, and CE. mtmcata ^ X CE. biennis ^ [Fj (M ? ) X Fj (B $ )] all gave 

 CE". biennis. 



GoLDSCHMTDT (33) investigated the cytology of these crosses and found a certain 

 amount of evidence for the conclusion that in the cross CE. muricata $ X <2l. biennis $ 

 the ovule nucleus of tE. muricata undergoes degeneration, its place being taken by 

 the pollen nucleus of (E. biennis. In the cross CE. biennis % X CE. muricata $ , the 

 CE. biennis nucleus is said to be destroyed and its place taken by CE. muricata. Thus 

 the hybrids with (£". muricata cytoplasm have nothing but CE. biennis chromatin, 

 while those with (E. biennis cytoplasm have notliing but (E. muricata chromatin. 

 If confirmed, these facts would fall in hne with the elimination described above, 

 with the difference that in this case the whole of the female portion of the zygote 

 would be eliminated, instead of a few chromosomes. 



We thus have a complete series from cases in which the male pronucleus degenerates 

 to a condition where the female substance is entirely elitfiinated : — 



1. Echinus ? X Mijtilus $ — Kupelwieser (48). The male pronucleus degenerates 



and development is carried on by the female pronucleus alone. 



2. Arbacia^ X Toxopneustes $ — Tennent (91). All paternal and some maternal 



chromosomes eliminated. 



3. Strongylocentrotus ? X Spkerechinus $ — Baltzer (6). Most of the paternal 



chromosomes eliminated. 



4. Echinus $ X Antedon $ — Godlewski (30). Early segmentation normal. No 



ehminatiou. 



5. Echinus acutus ^ X E. esculentus ^ — Doncaster and Gray (24). Some 



chromosomes eliminated (probably ? ). 



6. (E:. biennis % X <E. muricata $ — De Vries (99) and Goldschmidt (33). Tlie 



female pronucleus possibly entirely degenerating. 



