i8o COMPARATIVE ANATOMY 



dissolved, as also are the envelopes of the individual mother 

 cells of the daughter colonies. Thus each daughter group of 

 sixteen is set free, develops new cell envelopes and a new 

 colonial envelope, and forms a free-swimming colony. 



After repeated multiplication by this means a generation of 

 Pandorina colonies arises which enters upon a process of 

 conjugation before reproduction. Preparatory to this process 

 the sixteen individuals of a colony divide each into eight cells, 

 the mother colony ceases to swim, falls to the bottom, loses its 

 flagella, and the colonial as well as the individual membranous 

 envelopes gelatinise and are dissolved. The dissolution of the 

 envelopes proceeds slowly, so that it is some time before the 

 groups of eight cells, formed by division of the sixteen cells 

 composing the original colony, are set free. But eventually 

 each member of a group develops flagella and an envelope 

 and escapes as a single free swimming gamete. Two gametes 

 meet by their anterior ends and conjugate, fusing completely 

 with one another to form a zygote, which at first bears the four 

 flagella of the two united gametes. Presently the flagella are cast 

 off, the zygote surrounds itself with a thick envelope of red colour 

 and enters into a resting stage. Before it develops further it 

 is necessary that the zygote should be dried, which normally 

 takes place when the pool of rain-water in which it lives 

 becomes dried up in the summer. When the pool fills again 

 after rain the zygote develops. Its envelope thins out on one 

 side and forms a projection from the surface which breaks 

 through and allows the zygote to escape in the form of a naked 

 cell, which immediately develops flagella and an envelope, 

 divides into sixteen adherent cells surrounded by a colonial 

 envelope, and thus a new Pandorina colony is produced. 



It is not certain whether there is any differentiation between 

 the gametes of Pandorina. It appears that there are larger 

 and smaller colonies, producing larger and smaller gametes, 

 and also forms intermediate in size between these. It is stated 

 that the larger gametes never conjugate with one another, but 

 that the smaller and middle-sized gametes conjugate with the 

 larger and also with one another. If this is the case, there is 

 at the most the beginning of a differentiation into macrogametes 

 and microgametes, or, as we may say, into female and male 

 forms, in Pandorina. 



