POLYMORPHISM AND LIFE-CYCLES 165 



thereby becomes denser in consistence, but of less specific gravity. 

 Lastly, the cyst-membrane itself appears round the body, if it 

 has not already done so ; it generally stands off distinctly from the 

 surface of the body, and may vary in nature in different cases, 

 from a soft, slimy or gelatinous coat to a firm membrane of variable 

 thickness, often exceedingly tough and impervious. 



In the encysted state, Protozoa are able to withstand the many 

 vicissitudes to which they are naturally subject. They can then 

 be dried up, frozen, or sun-baked ; and since the protoplasm becomes 

 much lighter, they can be transported great distances by winds, 

 a fact which accounts for the appearance of Protozoa in infusions 

 exposed to the air in any situation a peculiarity from which the 

 name Infusoria is derived. In general the function of encystment 

 is to protect the organism against unfavourable conditions or violent 

 changes in the environment for instance, in freshwater forms, 

 against drought and climate, the cold of winter or the heat of a 

 tropical summer. In parasitic forms it is an adaptation commonly 

 connected with a change from one host to another. 



In parasites two types of cysts can be distinguished. In the 

 first place, full-grown forms may produce relatively large, resistant 

 cysts (Dauerzysten) of the ordinary type, almost invariably 

 spherical or ovoid in form. In the second place, the smallest forms 

 in the developmental cycle, the products of multiple fission or 

 " sporulation," may secrete round themselves tough, resistant 

 envelopes, within which they may multiply further ; in this case 

 the envelope is termed a sporocyst, and the entire body a spore* 



* The word " sporo " has come to be used in two distinct senses, as applied to 

 Protozoa, thereby producing a regrettable confusion and ambiguity. The word 

 itself is derived from the Greek <nr6pos, a seed, and was applied by botanists to 

 those cases where plants produce seed-like bodies which are not true seeds ; for 

 instance, the seed of an ordinary flowering plant is a complete embryo, with root 

 and shoot distinct, encapsuled in protective envelopes, but the " seed " of a fern 

 is merely a single cell enclosed in a protective membrane. Consequently the 

 term " spore " was used to distinguish the " seeds " of ferns, fungi, etc., from the 

 true seeds of flowering plants. 



It was observed at a very early period that many parasitic Protozoa produced 

 minute seed-like bodies, which conveyed the infection ; for those of Myxosporidia 

 Johannes Miiller coined the term " psorosperms," but in general the term " spore " 

 was used for these bodies, and the group in which the production of such spores 

 is a very characteristic feature was named the Sporozoa. 



With the progress of further investigation, it was found that in a great many 

 cases the essential part of the spore namely, the encapsuled protoplasmic body 

 arose by a process of multiple fission, hence termed " sporulation," from a larger 

 parent-body ; consequently the term " spore " has been used by many in a secon- 

 dary sense to denote a minute germ formed by multiple fission, as in the merozoites 

 of the malarial parasites. It is preferable to retain the word " spore " in its 

 original significance as a seed-like body contained in a resistant envelope or sporo- 

 cyst, and to use the word " germ " (equivalent to the German word Keim) for 

 the protoplasmic body formed by sporulation, whether enclosed in a sporocyst or 

 not. Unfortunately the word " germ " has become very much misused in popular 

 language, and a less ambiguous term would perhaps be the word gymnospore for 

 naked germs not enclosed in a protective envelope. 



There is no essential difference between a cyst and a spore, except their relation 



