322 ANIMAL DEVELOPMENT 



division to produce a number of relatively small free-swimming 

 bells, which are simpler in structure than the parent Animalcule 

 (fig. 844). The fate of these will be described in a later para- 

 graph. Spore-formation is, so to speak, an exaggeration of this 

 process, for where it takes place the body of the original form 

 breaks up into a large number of minute parts, each of which 

 is known as a spore. Such spores present a great variety of 

 characters in different cases, and often form part of a remark- 

 ably complex life-history, of which something will be said else- 

 where. They are developed within a firm capsule or cysi, with 

 which the parent form surrounds its body before beginning to 

 divide. 



The formation of spores is very characteristic of the parasitic 

 Animalcules known as Gregarines, which constitute the group 

 Sporozoa {i.e. Spore- Animals), and also of the Fungus- Animal- 

 cules (Mycetozoa). But for our present purpose it will be better 

 to take the simpler case afforded by a little bean-shaped organism 

 {Colpoda cticulkis) which often abounds in putrefying fluids. Its 

 body is uniformly covered by cilia, by means of which swim- 

 ming is effected. The life-history has been very carefully studied, 

 and some of the stages passed through are represented in fig. 846. 

 Before the body divides to give rise to offspring a firm case or cyst 

 is formed, but the subsequent events are not always the same. 

 Sometimes the body of the Animalcule, after losing its covering 

 of cilia, simply divides into two (b) new individuals, which once 

 more acquire cilia, and squeeze their way to the exterior through 

 a hole in the cyst. In other cases there is a division into four (c). 

 And lastly there may be spore-formation (d), which is a much more 

 complicated process. After formation of a cyst devoid of any 

 opening, the body of the Animalcule shrinks and becomes spheri- 

 cal, any food present within the body being expelled. The cilia 

 then disappear, and a second firm membrane is formed outside 

 the body, of which the substance now breaks up into a large 

 number of minute rounded spores enclosed in firm coats. By 

 rupture of the enclosing membranes the spores ultimately escape, 

 and the firm coat of each of these splits open to liberate the tiny 

 mass of livinor substance within. This at first resembles a minute 

 Amceba, feeding and crawling in similar fashion. One end then 

 grows out into a motile thread or flagellum, by which its owner 

 is able for a time to swim about. Later on, the body assumes 



