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AN INTRODUCTION TO MODERN GENETICS 



do not depend on differential divisions; in some organisms, in fact, 

 such as Protista and a few Metazoa of which Chaetopterus^ is the best 



Fig. 66. The Equality of Nuclear Division during Cleavage.— A a fertilized 

 newt's egg, lying in its gelatinous capsule, was constricted by a hair loop before the 

 first division. The zygote nucleus, in the right half, cleaved four times, and in the 

 16-cell stage one cleavage nucleus passed over into the left half of the egg. 

 6 both halves developed into normal larvae, showing that any one of the first 

 16 nuclei is capable of taking part in the formation of any organ. This result is only 

 obtained if the constriction divides the egg in such a way that both halves possess 

 part of the cytoplasmic organizer; a half lacking the organizer develops no 

 embryonic organs whatever nuclei it may possess. 



(After Spemann, from Huxley and de Beer.) 



^ Lillie 1902, Brachet 1937. 



