Echinoderm Hybridization. 131 



moment of fertilization has become enlarged, although it need not have 

 reached its maximum size. The most favorable moment for the dis- 

 placement of the course of heredity coincides with the stage of parthe- 

 nogenetic development in which the egg nucleus has reached its greatest 

 volume, so that (in part of the eggs) even a giving up of nuclear sap 

 to the cytoplasm may have taken place. If the eggs are fertilized at 

 this moment, resemblance to the mother follows. We may term this 

 the high point for displacement. 



Boveri, in his Zellen-Studien 5 and 6, has given us a nomenclature 

 for certain phenomena, which will be found useful: 



(i) The pronucleus in the egg = a hemikaryon. 



(2) The egg nucleus =a theh^karyon. 



(3) The sperm nucleus =an arrhenokaryon. 



Thus, all nuclei which arise from isolated egg or sperm nuclei are 

 hemikaryons. The first cleavage nucleus and its descendants are amphi- 

 karyons. Through reduction in oogenesis and spermatogenesis hemi- 

 karyons arise from the amphikaryons. A normal embryo arising from 

 the fertilized egg is amphikaryotic ; one from a fertilized egg without an 

 egg nucleus, i.e., from an enucleated egg, is arrhenokaryotic; one arising 

 from an artificially parthenogenetic egg is thelykaryotic. The two 

 latter are in the same sense hemikaryotic. So we may speak of amphi- 

 karyosis, hemikaryosis, etc. If the chromosomes of the first cleavage 

 nucleus have doubled without nuclear division we have a diplokaryon 

 and a diplokaryotic organism. Organisms which have in one region 

 normal nuclei, in another abnormal, containing only the derivatives of 

 an egg nucleus or of a sperm nucleus, are respectively partially-thely- 

 karyotic or partially-arrhenokaryotic. 



In Zellen-Studien 6 the nomenclature is slightly changed : 



1. Instead of hemikaryon we have monokaryon. 



2. A dikaryon or amphikaryon, as before. 



3. A trikaryon=one egg nucleus -f two sperm nuclei. 



4. A diplokaryon = tetrakaryon contains four times the ele- 



ments of the monokar3'on. 



Boveri had shown (Zellen-Studien 5) that the surface of the nuclei 

 of the somatic cells of the sea-urchin is directly proportional to the 

 number of chromosomes in the developing egg. Marcus (1906) had 

 shown the relation between temperature and nuclear size. With this 

 knowledge, Herbst proceeded to ascertain the fate of the sperm nuclei 

 in hybrids with altered heredity. He reasoned that if one compared 

 nuclei of parthenogenetic larvae with those of normal larvae and with 

 those of larvae having had the double treatment, the earlier nuclei of 

 the former, having half as many chromosomes, should have half as great 

 a nuclear surface as those of normally fertilized eggs, and therefore the 

 same should be time when compared with the nuclei of doubly treated 

 eggs. The matter, however, was not so simple. The study of the nuclei 



