GERM CELLS OF COELENTERATES 627 



If germ cells originate only from other germ cells, an animal 

 whose sexual individuals produce other sexual individuals by 

 budding might be expected to show a migration of germ cells 

 from the primary to the secondary individuals. If a migration 

 does not occur primordial germ cells must be present in the 

 budding zone, to satisfy the hypothesis mentioned (the germ 

 plasm theory). 



An examination of every possible path of migration in Hybo- 

 codon shows that such a migration does not occur. Careful 

 search of all the tissues of the medusae shows no trace of germ 

 cells, nor of cells in any way resembling germ cells, except in the 

 gonads. The bell and its tissues, the canals, the tentacle and its 

 base (the budding zone) all contain differentiated body cells, but 

 no germ cells. Germ cells in the secondary medusae must, there- 

 fore, arise quite independently of any germ cells of the primary 

 medusae. 



In developing buds the various tissues and organs of the 

 medusae are all present, at least as rudiments, before there is 

 any evidence of germ cells. The gonad is the last of the organs 

 to develop and no germ cells are present except in the gonad. 

 The medusa buds arise from differentiated cells of the tentacle 

 bulb, but before initiating the new growth they undergo a re- 

 gressive change and become indifferent or embryonic in appear- 

 ance and potency. The indifferent cells so formed eventually 

 undergo a re-differentiation and produce the various tissues and 

 organs, including the germ cells. Since the germ cells of devel- 

 oping medusae have thus come from differentiated body cells it 

 is clear that there is no distinct germ plasm in Hybocodon. 



In adult medusae only a few of the many oocytes of the gonad 

 grow and form ova, the other oogonia and oocytes being ab- 

 sorbed by the growing eggs as food. The germinal vesicle of the 

 growing egg remains very small and the nuclear reticulum 

 appears almost achromatic till the chromosomes begin to form. 

 An emission of chromatin from the nucleus takes place at the 

 beginning of growth. 



