ALTERNATION OF GENERATIONS. 



211 



The medusoid itself is a modified asexual growth, into which 

 some of the germ-plasma of the hydroid has migrated ; it is 

 literally only the bearer of the hydroid germ-plasma. Weis- 

 mann's classic researches on hydroids have shown that the 



I. The hermaphrodite fern prothaUus contrasted with (2 a) the male and (2 d) 

 the female thallus of li\er\vort, and (3 a and /') male and female prothaUus of 

 horsetail. Above are the corresponding reductions of the sexual prothallia 

 in (4) Salvinia, (5) Isoetes, (6) Cycad and Conifer, and (7) Phanerogam. 



reproductive cells, which by hypothesis bear the germ-plasma, 

 often arise far down in the hydroid body, and actually migrate 

 to their final seat in the bearer. Where the alternation is not 

 between sexual and asexual, but between the ordinary sexual 



