ni PARTHENOGENESIS 97 



The immediate cause of the retention of the second polar nucleus 

 within the egg seems to be that the tendency of the chromosomes to 

 form karyomeres (see p. 131) in Echinoderm eggs is accentuated, or 

 rather accelerated, so that the chromosomes begin their telophase 

 metamorphosis before the daughter plates of the second meiotic division 

 have fully separated. Consequently the two daughter nuclei come to lie 

 close together. 



The fertilization of Amphibian eggs by spermatozoa which have 

 been injured by radium emanation or by the action of certain poisons, 

 such as strychnine and nicotine, is in a sense intermediate between 

 artificial parthenogenesis and natural fertilization. G. Hertwig (1913) 

 established the, at first sight paradoxical, result that whereas if the eggs 

 of the common toad are fertilized by the spermatozoa of the frog, very 

 few zygotes result and these never develop so far as gastrulation, yet if 

 the frog spermatozoa be injured by subjection to radium emanation 

 before being added to the toad eggs, a large proportion of embryos pass 

 the hatching stage. The interpretation put forward is that in the first 

 case development is prejudiced by the incompatibility of the chromatin 

 of the two animals, while in the second case the frog spermatozoa are 

 so weakened that, though they enter the eggs, they have not the power 

 to fuse with the female pronucleus. Their entry, however, stimulates the 

 eggs to develop. This explanation is borne out by the further investiga- 

 tions of O. Hertwig (1913), who counted the haploid chromosome number 

 in newt larvae which had developed from eggs fertilized by spermatozoa 

 previously subjected to treatment with radium. 



These experiments are closely comparable to the cross-fertilizations 

 between Echinoderms, Molluscs, etc., described on p. 161. 



Nearly related to artificial parthenogenesis is the phenomenon of 

 merogony, the term applied to the fertilization of an egg fragment con- 

 taining no nucleus. Echinoderm eggs will survive being broken into 

 fragments, one only of which of course can contain the nucleus. The 

 non-nucleated fragments can be fertilized by spermatozoa and then 

 develop into dwarf and haploid, but otherwise normal, larvae. This 

 phenomenon is discussed in greater detail in Chapter VI. 



