338 Causes and Course of Organic Evolution 



In all, multiplication by gonidial or bud cells has been a 

 typical condition. In the aquatic genus Coleochcete most of 

 the vegetative cells of the thallus can become modified under 

 certain conditions for this purpose, when the entire contents 

 of each cell become rearranged, and a pair of cilia is formed. 

 The mass escapes by rupture of the parent wall, and, after a 

 free-sTN-imming stage, fixes down and di\ddes to form a new 

 plant. In connecting types between this and the Ricciese — 

 that became in large part palustral or sub-terrestrial — the 

 production of two cilia ceased, so that mainly from the lower 

 layers, more rarely from the rhizoids or from cells of the plant 

 body, bud cells or groups of bud cells formed, that detached 

 from the parent plant and reproduced it. 



Study of the sexual organs shows that numerous connecting 

 types have entirely disappeared. But the accompanying 

 diagrams suggest a graded series that very probably existed, 

 and which may have united genera related to Coleochcete with 

 others allied to the Riccieae. In C. scutata each antheridial 

 mother cell has its contents rounded off into a spherical or 

 oval mass, the spermatozoid, which develops two cilia, and 

 by rupture of the parent wall this escapes into the watdr. In 

 hypothetical forms now extinct, a stage doubtless existed 

 where each antheridial cell gave rise — as in some other living 

 green algae — to 8, 16, 32, or more spermatozoids. Each of 

 these spermatozoids seems also to have assumed a more elon- 

 gated shape for passage through the water, and even more for 

 penetration of the egg-cell and its accessory growths. 



But in gradual elaboration of the antheridium, and through 

 modification of what were once peripherally placed sperm 

 cells probably, the peripheral cells in such an antheridial body 

 ceased to mature as spermatozoids, and became protective 

 strengthening or bounding cells (fig. 12). This persists through- 

 out the whole of tlie Bryophyta and higher groups as the multi- 

 cellular antheridial wall (fig. 12, 3a-6a). The contained sperma- 

 tozoids became elongated and even curved in shape, at the 

 same time that they retained their cilia. From such a con- 

 dition to that seen in the Riccieae the transition is an easy 

 and progressive one, and is correlated with reduction in size, 

 and conversion into a thin coiled body, for the spermatozoid. 



The origin of the archegonium in the Hepaticae can be traced 

 with tolerable certainty from the simple oogonium of Coleo- 



