Echinoderm Hybridization. 133 



(2) Hybrids with small nuclei; from cultures with maternally 

 directed heredity: Out of 79 larvae, there were n with small nuclei. 

 A comparison with other nuclei shows that these are typical half nuclei. 

 We may distinguish two types of plutei of this class: 



(a) Plutei of the maternal type with small nuclei: A pluteus of 

 this type is shown in Herbst's (1907) fig. 10. Here only the female 

 nuclei have taken part in the development. The probable explanation 

 of this condition is that it is a case of partial fertilization and subsequent 

 separation of the two nuclei. In this event it must be acknowledged 

 that the sperm has been of some influence before its elimination. 



(6) Plutei of the paternal type with small nuclei: A pluteus of 

 this type is shown in Herbst's (1907) fig. 14. This is a typical arrheno- 

 karyotic pluteus, but shows evidences of its hybrid origin. The nuclei 

 must be designated as half nuclei. When regarded in the light of Boveri's 

 (1889, 1895) suggestion, to which we have already referred, it will be 

 seen that we have here larvae with only paternal nuclear material which 

 shows hybrid characters. These larvae are more properly partially- 

 arrhenokaryotic plutei. They have arisen from multiple fertilization. 



Displacement of heredity toward the maternal side occurs in many 

 ways. It is not essential that the processes of fertilization be modified 

 from the beginning. In some cases it is a delay of fertilization, i.e., 

 of copulation of egg and sperm nucleus, that is responsible for the dis- 

 placement. This delay may have various consequences: 



(1) It may lead to a failure of the sex nuclei to unite and in some 

 way to an elimination of the sperm. Thus half nuclei may originate 

 and give us wholly or nearly pure Splicer echinus plutei in the cultures 

 with maternally directed heredity. 



(2) It may lead to copulation after the egg nucleus has completed 

 the first steps of division. According as this is a dyaster or a monaster, 

 we may have two subdivisions; 



(a) The sperm nucleus may copulate with one of the two cleavage 

 nuclei. From such eggs partially-thelykaryotic larvae may arise. These 

 larvae are found infrequently. 



(6) The sperm nucleus may copulate with the egg nucleus after 

 the latter has reached a double size through monaster formation. Mon- 

 asters are common, so this case is not infrequent. It is to be expected 

 when a halo is present around the nucleus at the moment of fertilization. 

 There are probably other possibilities in methods of displacement. 

 Herbst believes that the cause of displacement lies in an altered ratio 

 of egg nuclear and sperm nuclear size. If the egg nuclear substance be 

 in excess, then there will be a preponderance of maternal characters, 

 while if the sperm nuclear substance be in excess there will be a pre- 

 ponderance of paternal characters in the descendants. 



Because of the great importance of this (1907) paper I give a brief 

 summary of Herbst's conclusions. 



(i) The critical stage for displacement occurs when the egg nucleus 

 at the moment of fertilization has shown a distinct increase in size. 



