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THE POPULAR SCIENCE MONTHLY 



certain influences have come from the nucleus of the egg, and perhaps 

 from the egg chromosomes. There is no doubt that most of the differ- 

 entiations of the egg cytoplasm have arisen during the ovarium history 

 of the egg, and as a result of the interaction of nucleus and cytoplasm ; 



CTENOPHORE 



ect. 



TURBELLARIAN 

 .ect 



mes. 



ECHINODERM 



ASCIDIANII 



end. 



eci 



ASCJDlAN II 



ect. 



AKNEIID 



end. 



Fig. 45. Types op Egg Organization in Difpeeent Phyla ; cross-hatched area, 

 mesoderm or mesenchyme (mes) ; horizontal lines, endoderm (end) ; ciear area, ecto- 

 derm (ect). In the first four figures the pattern of localization Is that which is 

 found at the close of the first cleavage ; in the annelid egg the localization of later 

 stages is projected upon the egg. 



but the fact remains that at the time of fertilization the hereditary 

 potencies of the two germ, cells are not equal, all the early stages of 

 development, including the polarity, symmetry, type of cleavage, and 

 the pattern, or relative positions and proportions of future organs, being 

 foreshadowed in the cytoplasm of the egg cell, while only the differ- 

 entiations of later development are influenced by the sperm. In short, 

 the egg cytoplasm fixes the general type of development and the sperm 

 and egg nuclei supply only the details. 



We are vertebrates because our mothers were vertebrates and pro- 



