VOLVOCALES 



25 



colonies are merely degenerate forms of the principal species, 

 G. pectorale. In the sixteen-celled colonies (G. pectorale) there are 

 four cells in the centre and twelve in the periphery, each cell being 

 surrounded by a gelatinous wall and fused to the neighbouring 

 cells by means of protrusions, whilst the protoplasts of the indi- 

 vidual cells are also united by fine protoplasmic threads. The ovoid 

 or pyriform cells contain contractile vacuoles and are provided with 

 a pair of flagellae. The centre of the colony is composed of mucus 

 and there is also a firm outer gelatinous layer. The shape of the 

 colony accounts for its mode of progression which is by means of 

 a series of somersaults around the horizontal axis. At asexual 

 reproduction all the cells divide simultaneously to form daughter 

 colonies. If single cells should become isolated then after a time 

 they will give rise to {a) daughter colonies, {h) akinetes, or (c) a 

 palmelloid state. Sexual reproduction is by means of naked iso- 

 gametes, fusion occurring between gametes from separate colonies 

 as the various species occur in + and - strains. The resulting 

 quadriflagellate zygote soon comes to rest and subsequently germi- 

 nates, when it gives rise to four biflagellate haploid cells which are 

 liberated together as a small colony. When the later development of 

 these cells is followed it is found that two of them give rise to + 

 and two to - colonies, suggesting that meiosis must take place at 

 germination of the zygote. 



*CHLAMYDOMONADACEAE:P^wJorma (after Pandora's box). Fig. 16. 



The colonies are oblong or spherical and are composed of four, 

 eight, sixteen or thirty-two cells, sixteen being the normal number 

 in the common species P. morum. The cells, which are arranged 



A ' B 



Fig. 16. Pandorina morum. A, vegetative colony ( x 975). B, colony with female 

 gametes ( x 975). (After Smith.) 



