74 



ALTERNATION IN THE THALLOPHYTES 



a development stands hitherto alone, 1 but biologically it occupies the 

 same place as a simple sporophyte. 



FIG. 42. 



Coleocha.ctepulvina.ta, after Oltmanns. i, Young zoosporangium (?). 2, 3, A 

 4, Oogonium shortly before opening. 5, Ditto after 



Antheridia 



(a) and young oogonium (p). 4, Oogonium shortly before opening. 5, Ditto after opening. 

 6, Zygote still with two nuclei. 7, Zygote developed to " fruit." 8, Germinating hypno- 

 zygote. a, Antheridium. 0, Oogonium. sk, Male nucleus, ek, Female nucleus, ckr, 

 Chromatophore. py, Pyrenoid. k, Nucleus. 



Though the cases are thus seen to be still comparatively few in which 

 the Thallophytes have had their cytological condition traced throughout 

 the whole course of the individual life, there is a growing body of evidence 

 to show that an obligatory alternation of cytologically distinct generations 



1 It is possible that the multicellular spores of Ascomycetes supply a parallel. If, 



as is probable, the reduction accompanies the formation of the ascospores, then the 



subsequent divisions in those spores would hold a similar place in the cytological cycle 

 to those in the fruit of Coleochaete. 



