THE HEREDITARY FACTORS AND DIFFERENTIATION 405 



pyramidal form, the arms being supported by simple rods. The 

 pluteus of Sphaerechinus granulans is of a more rounded form, with 

 two of its four arms longer than the others : the skeleton is in the 

 form of a rough framework made up of several rods interconnected. 

 The hybrid obtained by fertilising eggs of Sphaerechinus with 

 sperm of Echinus is intermediate in shape between the parental 

 types, and its structures show some of the characteristics of both 

 parents.^ Analogous results have been obtained from a study of 

 hybrids between fish species.^ It is clear that those characters in 

 which a hybrid resembles its father are due to paternally inherited 

 genes. 



In heteroplastic experiments in which a piece of tissue from an 

 embryo of one species is grafted into an embryo of another species, 

 artificial embryonic or larval chimaeras are produced. When the 

 two species are closely related, as are for instance Triton cristatus 

 and Triton taeniatus, the result is the production of fairly normal 

 embryos.^ Chimaeras may also be formed by mixing regeneration- 

 buds of the black and the white varieties of the axolotl* (fig. 195). 



In all such cases, when the operation is performed before irre- 

 versible determination of the tissues has taken place, the general 

 pattern of diiferentiation is imposed by the field-system of the 

 organism or region, acting as a unit. But the detailed peculiarities 

 of the differentiated tissues are determined by the hereditary con- 

 stitution of the species to which the tissue originally belonged. This 



^ A related yet separate problem is the question as to the relative importance 

 of the parts played by nucleus and cytoplasm in controlling the development of 

 the larval hybrid. The method used to investigate this matter has been to fertilise 

 enucleated eggs w^ith foreign sperm. Experiments of this kind have been per- 

 formed on Amphibia (Baltzer, 1920), where, however, the embryos do not live 

 long enough to enable definite conclusions to be drawn, and on Echinoderms, 

 where until recently the technical difficulties involved have introduced un- 

 certainties, particularly as to whether the nucleus really is eliminated from the 

 egg. These difficulties have now been overcome, and it appears that the cytoplasm 

 of an enucleated egg can exert some effect on the characters of the larva, although 

 the nucleus seems to be more powerful (Horstadius, 1932). The presence of heredi- 

 tary factors in the cytoplasm of the oocyte has been revealed in experiments on 

 sex-determination in moths and on the inheritance of dextrality in snails, and in 

 each case there is reason to believe that these factors are the persistent results of 

 genes situated in the chromosomes at a previous stage. The same may be true in 

 the case of the Echinoderm hybrids just mentioned. See also Boveri, 1903. 



^ Newman, 1914. ^ Spemann, 1921. 



* Schaxel, 1922 a. 



