THE CONSERVATION OF LIFE 245 



of abundant food they commence almost simultaneously to 

 multiply by successive fission. We have now a colony of sixteen 

 Pandorinae, each consisting of sixteen cells. For some time the 

 young animalculse are held together by the old common gelatine- 

 cover, but very soon the daughter-colonies leave the maternal 

 home and go into the outer world. 



They are delicate creatures whose existence is only too often 

 threatened by injurious external influences. How are they to 

 preserve their kind when the waters in which they live dry up, 

 or severe cold freezes them to ice ? Like the amoebae and many 

 other protozoans, the Pandorina is assisted through the hard 

 times by a rest stage. Sometimes the body-walls of a colony 

 divide each into eight flagellate parts, the so-called gametes. 

 When these have reached the water, gametes of different descent 

 proceed to pair. The products of these fusions live free only 

 for a short time ; then they encyst and their flagellae become 

 degenerate. Whatever may happen now, the young germ pro- 

 tected by the cyst is able to withstand all perils, calmly awaiting 

 better times. As soon as conditions are once more favourable 

 it crawls forth from its prison and by fission develops once more 

 a young colony of sixteen heads. Thus we find in the Pandorina, 

 in addition to asexual, a sexual reproduction, but the latter is 

 still of a very simple character, for a differentiation of male and 

 female sex has not yet been reached. 



A near relation of Pandorina is Volvox which lives in close union 

 with it. Every lover of Nature knows the little greenish balls, 

 easily noticeable with the naked eye, which slowly rotate through 

 the water. While the body of the Pandorina consists only of 

 sixteen cells, a Volvox colony may consist of more than ten 

 thousand cell-individuals which form the one-layer wall of a large 

 hollow sphere filled with a thick fluid. The union of the indi- 

 vidual citizens of this cell-state has become most intimate. Here, 

 too, the entire colony is covered with a uniform gelatine shell, 

 but the different individuals are in addition connected with each 

 other by thin bridges of protoplasm. As in Pandorina, so the 

 single cells possess here still great independence. Like every 

 other cell, they consist of protoplasm and nucleus, but in addition 

 we find some which are furnished with contractile vacuoles, 

 chlorophyll, pigment, flagellae, and a special close-fitting gelatine 



