GERM CELLS OF COELENTERATES 31 



plasticity in the cells of the body, a regressive change to an 

 embryonic condition preceding the formation of the bud. 



The germ-plasm theory invokes the aid of latent germ cells 

 to account for regeneration, but there is no evidence of this in 

 Hydrozoa. So many cases are recorded, in many groups of 

 animals including vertebrates, of the de-differentiation of tissue 

 cells and the formation of the regenerated structures from an 

 indifferent or embryonic mass of cells, that it may be doubted 

 whether regeneration is ever related to germ cells. When 

 coelenterate tissues are ground up and the cells isolated, the lat- 

 ter coalesce to form masses capable of regenerating complete and 

 normal individuals, but in all such masses the cells have become 

 despecialized before the regenerative processes begin. The ob- 

 servations upon dissociated cells of hydroids show that germ 

 cells, if present, degenerate and play no part in the ensuing 

 regeneration, while the body cells, under the same stimulus, lose 

 their specificity, become totipotent, and produce the variously 

 specialized cells and differentiated structures of the normal 

 individual. 



Many animals of different phyla are known whose gonads are 

 present at the breeding season and entirely unrecognizable at 

 other times, in such cases the germ cells arise from the body 

 cells of the appropriate region. Recent work upon mammals 

 gives strong evidence of the degeneration of all germ cells formed 

 during embryogenesis, the definitive germ cells only differen- 

 tiating after birth from the germinal epithelium of the gonad. 



Explanted tissues, grown in culture media outside the body, 

 may undergo a de-differentiation and form cells more or less 

 embryonic in character. Cancerous growths, originating from 

 tissue cells, display a capacity for long-continued and apparently 

 indefinite growth and division. Such facts are indicative of a 

 less definite distinction between germ cells and body cells than 

 has usually been maintained, and the possession of a considerable 

 capacity in specialized cells to undergo a further differentiation, 

 even in a new direction. 



The investigations discussed in this section furnish a great 

 body of facts utterly inconsistent with the theory of the con- 



JOURNAL OF MORPHOLOGY, VOL. 33, NO. 1 



