208 HEREDITARY TRANSMISSION [CH. 



fragments of the eggs of one species (S'phaer echinus granu- 

 laris) by the sperm of another (Echinus microtuberculatus). 

 He shook up the eggs in water in a tube till they were broken 

 up, and picked out under the microscope non-nucleated 

 fragments to which Echinus sperm was added. Some of 

 these fragments segmented and developed into larvae which 

 in their skeletal characters at least were of the pure Echinus 

 type. If there are no sources of error, this experiment proves 

 that some characters at least are determined purely by the 

 nucleus. Objection, however, has been taken to it on various 

 grounds chiefly that the ordinary hybrids of these species, 

 made from nucleated eggs, may at times have the Echinus 

 type of skeleton. BOVERI, however, in his control experi- 

 ments never got hybrid larvae showing no S'phaer echinus 

 characters, and the experiment may probably be taken as 

 proving that the skeletal characters of Echinoid larvae are 

 determined by factors contained in the nucleus. 



Additional evidence is provided by the experiments of 

 HE REST. He treated the eggs of Sphaer echinus with a fatty 

 acid that induces artificial parthenogenesis, and then, before 

 segmentation began, fertilised them with sperm of Strongy- 

 locentrotus lividus. By this procedure the nucleus in some 

 eggs was caused to divide before the sperm nucleus was 

 ready to conjugate with it, and the sperm nucleus was car- 

 ried into one of the first two blastomeres, and there conju- 

 gated with its nucleus. As a result, one of the first two 

 blastomeres contained a nucleus of purely maternal origin, 

 and containing the reduced number of chromosomes, while 

 the other was hybrid, and contained both paternal and 

 maternal chromosomes. Among the plutei reared from eggs 

 treated in this way, some were found in which one half 

 of the body had purely maternal skeletal characters, while 

 the other half had the characters commonly found in 



