144 DOCTRINE OF EVOLUTION 



of hereditary qualities. In reproduction, the male 

 sex-cell, which is scarcely more than a minute mass of 

 chromatin provided with a thin coat of protoplasm and 

 a motile organ, fuses with the egg, and the nuclei of 

 the two cells unite to form a double body, which con- 

 tains equal contributions of chromatin from the two 

 parental organisms. This gives the physical basis 

 for paternal inheritance as well as for maternal inheri- 

 tance, and it shows why they may be of the same or 

 equivalent degree. 'V\Tien, now, the egg divides, at 

 the first and later cleavages, the chromatin masses or 

 chromosomes contained in the double nucleus are split 

 lengthwise and the twin portions separate to go into 

 the nuclei of the daughter-cells. As the same process 

 seems to hold for all the later divisions of the cleavage- 

 cells whose products are destined to be the various 

 tissue elements of the adult body, it follows that all 

 tissue-cells would contain chromatin determinants 

 derived equally from the male and female parents. 

 As of course only the germ-cells of an adult organism 

 pass on to form later generations, and as their content 

 of chromatin is derived not from the sister organs 

 of the body, but from the original fertilized egg, there is 

 a direct stream of the germ plasm which flows contin- 

 uously from the germ-cell to germ-cell through succeed- 

 ing generations. It would seem, therefore, that the 

 various organic systems are, so to speak, sister products 

 in embryonic origin. The reproductive organs are not 

 produced by the other parts of the body, but their cells 

 are the direct descendants of the common starting- 

 point , namely, the egg. As the cells of the reproductive 

 organs are the only ones that pass over and into the 



