200 THE PHYSIOLOGY OF REPRODUCTION 



that the chromosomes seen at the poles of the spindle at the 

 termination of mitosis are individually identical with those 

 seen at the equator of the spindle at the next mitosis. He 

 points out, further, that there is distinct evidence that this is 

 not the case in certain Protozoa and Coelenterata. Again, 

 Hickson has called attention to the long duration of the period 

 of conjugation in Infusoria (Heterokaryota), remarking that 

 this is difficult to explain if we accept the view that the cyto- 

 plasm of the conjugating cells is not concerned with the trans- 

 mission of hereditary characters. 1 



Verworn, in the criticism referred to above, has objected 

 on more general but scarcely less substantial grounds to the 

 doctrine that the hereditary transmission of parental char- 

 acteristics is mediated by the transference of nuclear substance 

 only. ' The physiological mode of thought will hardly be 

 able to adapt itself to the idea of a single hereditary 

 substance, which is localised somewhere in the cell, and 

 transferred in reproduction. A substance that is to convey 

 the characteristics of a cell to its descendants, before all else 

 must be capable of life, i.e. must have a metabolism, and this is 

 impossible without a connection with other substances necessary 

 to cell-metabolism, i.e. without the integrity of all essential 

 cell-constituents. The designation of a single cell-constituent 

 as the specially differentiated bearer of heredity is wholly un- 

 justified ; the cell protoplasm is of exactly the same value in 

 this respect as the nucleus, and we must constantly return to 

 the fact that in all living nature no instance is known in which 

 a complete cell possessing nucleus and protoplasm does not 

 always mediate hereditary transmission. The character of 

 every cell is determined by its peculiar metabolism. Hence, if 

 the peculiarities of a cell are to be transmitted, its characteristic 

 metabolism must be transmitted ; this is only conceivable when 

 nuclear substance and protoplasm, with their metabolic relations, 

 are transferred to the daughter-cells. This is true of the sexual 

 reproduction of the higher animals, as well as of the asexual 



1 Hickson, " The Physical Basis of Inheritance," British Assoc. Reports, 

 Leicester Meeting, 1907, and Trans. Manchester Micr. Soc., 1907. See also 

 Fick, " Vererbungsfragen Reduktions und Chromosomen hypothesen Bastard- 

 regeln," Merkel und Bonnet's Ergeb.f. Anat. u. Phys., vol. xvi., 1906. 



