270 The American Naturalist. [April, 



is the process of tissue differentiation which emphasizes the 

 contractile power in the muscle cell, at the same time limiting 

 and finally eliminating the morphogenic power, and which 

 gives the sex cells morphogenic power in such marked degree 

 while it deprives them largely of contractile power. In a 

 former paper, 6 I stated this view thus: "The segmentation 

 nucleus of metazoa contains, as in the infusoria, both micro 

 and macro nuclear elements, but these are retained in varying 

 proportions in its descendants, i. e., in the cells of the adult 

 organism. Through a process of division of labor the power 

 of rejuvenescence becomes restricted to comparatively few of 

 the cells derived from the segmentation nucleus." 



While Minot's views are in part borne out by the conditions 

 in Cymatogaster, the italicised part of the quotation below 

 finds no support, and is negatived by all the observations made 

 in Cymatogaster. His conclusion, as translated by me, is : 

 " Somatic cells are simply cells in which the activity of hered- 

 ity is prevented by senescence, viz. : tissue differentiation, but 

 the somatic cells can, under favorable conditions, be \ 

 the rejuvenated stage and then develop the most complete or, at least, 

 more compb tr, hereditary power." 



The sex cells originally segregated retain their individ- 

 uality, but undergo a measureable change between the time of 

 their segregation and 7 mm. long larvae. Soon after the 

 larva has reached a length of 7 mm., the sex cells begin to di- 

 vide. In the meanwh ile they have migrated laterad and lie, for 

 the most part, in a longitudinal groove formed by a duplica- 

 tion of the peritoneum into which a few peritoneal cells have 

 also migrated. In one such case an extra sex ridge rvas formed 

 much f > rt her forward than usual, in connection with a few sex cells 

 which were accidentally belated in their migration. The peri- 

 toneal cells which have migrated into the sex ridge give rise 

 to the entire stroma of the future sex glands, and together 

 with the sex cells form a core quite distinct from the covering 



