Chapter XVI . 



THE INHERITANCE MATERIALS OF GERM-CELLS 

 INITIATORS RATHER THAN DETERMINERS 



Antecedents of the Cytoplasmic and Nuclear Theories of 



Inheritance Material 



FOR the purpose of calling vividly to mind the character 

 of the evidence on which the two propositions rest 

 with which the last chapter ended, it will be profitable to 

 cast a glance back on the course along which biology has 

 come down to us, with a view to finding a shai'ply out- 

 standing spot in the early growth of knowledge which led to 

 each of them. On the botanical side such a spot in the 

 knowledge of cytoplasm as hereditary substance, is the 

 work of Schleiden on the microscopic structure of adult 

 and developing plant tissues. The publication of his Ueher 

 Phytogenesis, 1838, may be taken as the starting point of 

 our knowledge of cellular transformation in the production 

 of tissues. It should be remembered that the observers of 

 that period had very hazy notions about the distinction 

 between nucleus and cell-body, or cytoplasm. On the 

 zoological side the publication by Ehrenberg, in 1836, of 

 Die Infusionsthierchen als volkommene Organismen may, I 

 think, be looked upon as the first milestone in the progress 

 of knowledge of cytoplasmic transformation into tissue sub- 

 stance. 



The ever-broadening stream of knowledge of the chromo- 

 somes in relation to heredity is usually held to have orig- 

 inated in the discovery forty years ago, by O. Hertwig, 



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