i84 CHLAMYDOMONAS, VEGETATIVE REPRODUCTION 



described recurs about every twenty-four hours, one individual 

 would in the course of a week give rise to 2,097,152 ! Hence the 

 often rather sudden appearance of such organisms in huge 

 numbers in small pieces of water. It is to be noticed that the 

 protoplasm of the parent is entirely incorporated in the bodies 

 of its offspring, the dead cell-membrane alone remaining 

 behind ; but for the destruction of a large proportion, such 

 organisms might be regarded as potentially immortal, in the 

 sense that death from senile decay is unknown. 



Under certain undetermined conditions the daughter-indi- 

 viduals fail to produce cilia and remain at rest within the 

 parent cell-membrane, which gradually becomes mucilaginous 

 (Fig, 96, c). The daughter-individuals assimilate and grow 

 and sooner or later divide again, their membranes in their turn 

 becoming mucilaginous. This may be repeated indefinitely 

 until large gelatinous masses, enclosing numerous cells and known 

 as Palmella-stages (Fig. 96, b), are produced. In the temporary 

 adoption of this sedentary mode of life, Chlamydomonas and 

 similar organisms exhibit a more marked resemblance to the 

 majority of plants. On the return of favourable conditions, 

 the individual cells acquire cilia and, escaping from the en- 

 veloping mucilage, resume the motile condition. 



For a long time multiplication may be purely vegetative, 

 but sooner or later — usually when growth is checked by a 

 deficiency in nutritive salts — another method of reproduction 

 sets in. This too is accompanied by division, but the resulting 

 segments are more numerous, 16 or even 32 being formed ; 

 these are liberated as sexual cells or gametes (Fig. 97, a), which 

 only differ from ordinary individuals in being considerably 

 smaller and, in most species of Chlamydomonas, naked {i.e. 

 devoid of a cell-wall). They move for a short time, but soon 

 meet in pairs, whose cilia become entangled, and thereafter a 

 gradual fusion (Fig. 97, b and c) of the two protoplasts and of 

 their nuclei (Fig. 97, _;', A') takes place. There results a single 

 cell (termed a zygote) which moves for a brief period with the 

 aid of its four cilia (Fig, 97, d) and then comes to rest. The 

 cilia are withdrawn, the protoplast assumes a spherical shape 

 and secretes a thick stratified membrane, and large quantities 

 of a reddish-yellow oil appear in the cytoplasm. The refulting 



