626 SCIENCE PROGRESS 



After a period of movement, which is of varying duration 

 (generally several hours), the zoospores come to rest and the 

 cilia are drawn in ; they become attached to some dark object 

 by their pointed front end, a cell-wall is excreted and cell- 

 division sets in, whereby a new organism is produced. In the 

 simplest unicellular green Algae, which are motile during most 

 of their life-history, the zoospore does not differ except in size 

 from the mother-cell, and becomes enveloped by a cell-wall 

 before it is liberated from the latter (cf. the description of 

 Chlamydomonas on p. 628). In the higher forms it often 

 happens — either as a normal phenomenon or under certain 

 exceptional conditions — that after contraction and division of 

 the contents of the zoosporangium has taken place, these 

 contents are not liberated as zoospores, but immediately become 

 enveloped by (generally thick) cell-walls. The reproductive cells 

 thus formed are obviously a mere modification of the ordinary 

 zoospore and are known as aplanospores ; they are generally 

 capable of passing through a prolonged resting-period before 

 germination takes place. 



In the sexual reproduction of the green Algae we have to 

 distinguish between cases in which the sexual cells (gametes) 

 are alike (isogamy) and cases in which they are unlike 

 (anisogamy). In the isogamous forms the sexual cells are 

 quite similar to the zoospores except that they are of smaller 

 size and are consequently produced by a larger number of 

 divisions in the cell-contents of the mother-cells (so-called 

 gametangia) than is the case in zoospore-production. The 

 perfectly identical gametes of these isogamous forms meet in 

 pairs during their movement (fig. 1, f) and fuse together to form 

 a resting spore, which becomes enveloped by a thick membrane 

 and is known as a zygospore. In some of the forms showing 

 this method of sexual reproduction there are slight differences 

 in size and power of movement between the two fusing gametes; 

 these slight indications of anisogamy are much more pronounced 

 in other forms, and thus we can trace nearly all stages from 

 typical isogamy to that extreme of anisogamy which goes under 

 the name of oogamy and is found only in the highest types of 

 each series of algal forms. Here we have to distinguish 

 between a female cell or ovum, which is generally a large more 

 or less spherical and deeply coloured motionless cell, mostly 

 remaining in the female organ or oogonium until liberated from 



