330 



EVOLUTION AND GENETICS 



fertilization need not be an extreme one. Loeb's researches in 

 this field showed that a slight increase in the potassium or sodium 

 chloride concentration of sea water was enough to bring about 

 parthenogenetic development of the eggs of sea urchins and 



Soma of stem-mother and 

 migrant females (her daughters) 



Parthenogenetic eggs 



of migrant females 



giving rise 



to 



Anaphase of the single 

 maturation division 



Primary 

 Spermatocyte 



Secondary 

 Spermatocytes 



i'ene' 



Spermatids 

 a 



Fig. 188.— The chromosome cycle in the parthenogenetic insects, aphids and 



phylloxerans. (From Waher.) 



annehd worms. He further records that the moderate agitation 

 caused by transferring the eggs of some starfishes from one dish 

 to another was suflacient to cause cleavage without fertilization. 

 Such cases could be multiplied from the findings of other investi- 

 gators, but there is little variety in them. Development proceeds 

 to various degrees, but in Loeb's researches the chemical stimulus 



