SEXUAL REPRODUCTION 319 



either thus or side to side. In Chlamydomonas media the 

 equal gametes have walls, and before union the plasma 

 contracts and escapes from the wall. In Chlamydomonas 

 Braunii the two gametes come together while still enclosed 

 in walls ; the contents of the one slip inside the wall of 

 the other, where fusion takes place. Here we have a differ- 

 ence in the behaviour of the two gametes. In Chlamydo- 

 monas coccifera a definite ovum is formed by a vegetative 

 cell casting its cilia and increasing in size. Sperms are 

 formed by the internal division of another cell in the more 

 usual way. Compared with the egg they are quite small 

 and they are motile. The egg is fertilised by one of the 

 sperms. Here we have a difference in structure. 



Within the bounds of a single primitive genus we have, 

 then, an advance from the pairing of equal gametes, 

 isogamy, to the fertilisation of a large egg by a small motile 

 sperm, oogamy. Similar series may be traced in other 

 families of the green algae, e.g. in the Volvocaceae and the 

 Ulothricaceae. What we wish here to emphasise is that 

 while the fundamental characteristic of sexual reproduction 

 is the fusion of two gametes, the trend of evolution is towards 

 the differentiation of a large motionless egg and a small 

 motile sperm, though the motility of the sperm has been 

 subsequently lost, for secondary reasons, in the higher 

 members of the land flora. 



In many of the higher algae a new feature is found in the 

 retention of the egg in organic connection with the parent 

 plant, and this condition is constant in the land flora, where 

 the egg is produced in a special organ, the archegonium, 

 which from its universal occurrence in all forms in which the 

 reduction of the gametophyte has not resulted in its oblitera- 

 tion, must be taken to be a structure very favourable to the 

 nourishment and protection of the delicate egg cell. The 

 enlargement of the egg may be regarded as advantageous in 

 making possible an accumulation of reserve food substance 

 with which the zygote may start its career. The retention 

 of the egg in the parent plant makes for its better protection 

 and nourishment and also benefits the zygote. 



