THE GREEN ALG.E 627 



the latter by its decay, and a many times smaller almost colour- 

 less or frequently yellowish male cell or spermatozoid, which 

 is actively motile and obtains access to the ovum within the 

 oogonium by a special aperture, which is formed in the wall 

 of the latter. The oogonium is generally a somewhat enlarged 

 cell, characterised by its ventricose appearance, and as a rule 

 the whole of the contents contract away from the wall to 

 form but a single ovum. The spermatozoids, which generally 

 show some resemblance to the zoospores of the same Alga, 

 but are much smaller and have at the best but a minute 

 chloroplast, are formed generally in considerable numbers in the 

 male organ or antheridium. The fertilised ovum, after becoming 

 surrounded by a thick wall, constitutes a resting oospore. 

 Zygospores and oospores may on germination either give rise 

 to a new organism straight away, or may first form a number 

 of zoospores, which on coming to rest produce the mother- 

 plant. 



The brief outline given above will suffice to indicate the 

 general features of the group, the discussion of whose phylogeny 

 will occupy us in the following pages. 



The subject cannot be more worthily opened than by a 

 brief consideration of the genus Chlamydomonas (Goroschankin 

 29 and 30, Dill 21, Dangeard 16 and 18, Klebs 40), which may 

 without exaggeration be said to have played the most important 

 part in the advances of the last fifteen years. The genus com- 

 prises a large number of species, most of which inhabit small 

 stagnant pools or larger areas of fresh water, while a few are 

 marine. The individuals (fig. 1, a) are unicellular, more or less 

 spherical or ovoid in shape, and are in many cases provided with 

 a pronounced kinoplasmic protrusion (Membranwarze) at the 

 front end, from which the two equal cilia arise (four cilia in 

 the allied genus Carteria). The cell-contents include the very 

 characteristic bell- or basin-shaped chloroplast, the thickened 

 floor of which contains one (sometimes two) spherical or 

 sausage-shaped pyrenoids (p ; the allied genus Chloromonas 

 (Wille 69) differs in lacking pyrenoids), a central nucleus, and 

 two contractile vacuoles (v), which pulsate alternately, and are 

 situated beneath the point of attachment of the cilia. Abundant 

 starch is formed as the result of photosynthesis. A dot-like or 

 linear eye-spot (s) is situated laterally, but generally nearer 

 the front end of the cell, and is apposed to the chloroplast ; 



