254 



INTERNAL FACTORS 



IV. i 



premising only one thing 1 , namely, that both maternal and 

 paternal nuclei are certainly not necessary for the production of a 

 new individual, however beneficial it may be for the elements of 

 two parents to be commingled in the body of one offspring. In 

 artificial parthenogenesis an egg is stimulated to development by 

 a solution, and produces a normal larva ; this creature possesses 



nuclei of maternal origin. The 

 converse case is seen in the 

 phenomenon of ' merogony '. 

 An enucleate egg-fragment, 

 fertilized with sperm of the 

 same species, gives rise to a 

 new organism, provided only 

 with paternal nuclei. 



The older view that the 

 union of the pronuclei was 

 the essential act in fertiliza- 

 tion can no longer be held, 

 for, certainly, both nuclei are 

 not necessary. The most, 

 therefore, that can be said 

 will be that a complete set of 

 the chromosomes (or smaller 



chromatin units) of the species 

 FIG. 154. On the left the pro tozoon < i-w , , 



Stentor, cut into three pieces, each con- ls necessary for differentiation, 

 taining a piece of the nucleus. On "Whether this is so or not we 

 the right, each piece has regenerated fc ; ire 



the missing parts and become a com- t * 



plete Stentor. (After Gruber, from 1. It will of course be al- 

 Korschelt and Heider.) lowed that the nucleus is neces- 



sary for the life of the cell and to a certain extent for the assump- 

 tion of form. The numerous and familiar experiments on Protozoa 

 have sufficiently established this point. 1 A nucleate piece will live 

 and regenerate lost parts, an enucleate piece will not (Fig. 154). 

 2. It will also be admitted that the maturation processes in 

 the germ-cells of the two sexes are extraordinarily alike and 

 end in the reception by each germ-cell of one-half the normal 



1 See especially M. Verworn, Die physiologische Bedeutung des 

 Zellkerns, Pfttiger's Arch, li, 1892 ; in this paper an account of the work 

 of Gruber and others will also be found. 



