70 LECTUEES ON BIOLOGY 



organism, and that there is therefore no justification for retain- 

 ing the belief in the existence of such ' simplest organism.' 

 When and whether Monera lived in times past can only be 

 a matter of speculation. 



The size of the different species of cells varies between fairly 

 wide limits. By far the larger majority is minute, standing close 

 to the limits of visibility. Often it is just barely possible to 

 observe them with the naked eye or a magnifying glass as minute 

 points. But there are cells so incomprehensively minute that 

 they can be observed only with the most powerful instruments, 

 magnifying 2,000 to 3,000 times. To these infinitely minute 

 bodies belong numerous bacteria, several protozoa, and the male 

 germ-cells of many animals. Some of these protozoa and bacteria 

 attain only a length of ^^ mm. The cocci of pus attain a 

 volume of only 1 700>0 1 00 000 c.mm. In one drop of water there is 

 ample room for thousands of millions of these bacteria, and a 

 glass of water would to them be more immense than the earth 

 is to the human race. In spite of this minuteness, attempts 

 have been made to calculate the weight of one of these cocci ; 

 the result given is e^b~6To"oo^ob m - ^ n or der, therefore, to obtain 

 1 gramme of them it would be necessary to heap up six billions 

 of these minute organisms. But even with these minute 

 measures the lowest limit of life has apparently not yet been 

 reached, for the germs of pleuro-pneumonia, for instance, are 

 so minute that they pass even through the densest porcelain 

 filter; indeed, there seems little hope of our ever perceiving them 

 with modern microscopes ; we know the effects of these dan- 

 gerous organisms, but the organisms themselves remain invisible. 



Many cells, on the other hand, reach considerable dimensions, 

 and may be easily perceived with the naked eye. Veritable 

 giants among them are the eggs of different animals, in particular 

 of reptiles and birds, for they are nothing but single enormous 

 cells. It must, however, be taken into consideration that by 

 far the greater part of these eggs consist of the store of food 

 which is to serve later for the support of the growing embryo. 



In plants, too, we know cells of considerable magnitude, as, 

 for instance, the bast-cells, which reach a length of 10 to 20 cm. 

 Caulerpa, one of the marine Algae, though it often reaches a 

 length of 1 metre and has a body with stem and leaves, is 

 nevertheless but a single multi-nucleated cell. 



