THE FOR.MATIOX OF GERiM-CELLS 



187 



and hundreds or even thousands of cell-generations are passed 

 through from the fertilised egg-cell onwards before they appear. 

 In species which exhibit a complete alternation of generations, 

 they are first formed in particular parts of the medusae which 

 have arisen from the polype-stock by budding — usually in the 

 ectoderm of the manubrium. No trace of them is to be seen in 



D 



Fig. 12. — Diagram of the degeneration of the Medusa into a mere gonophore. 

 A, Medusoid bud; B, a Medusa shortly before it is set free; C, degenerated 

 Medusa, in which the manubrium is present, but the mouth and tentacles are 

 wanting; D and E, further stages in degeneration. (From Hatschek's ' Lehrbuch 

 der Zoologie.') 



the young bud, and in many cases they only become differentiated 

 from the other ectoderm-cells after the medusa has become de- 

 tached from the stock, and has developed into an independent, 



