24 POPULAR ILLUSTRATIONS OF 



of the development a " cyst membrane " (d, Fig. 15, p. 22) is thrown 

 round the cell to strengthen it, and enable it the better to hold its 

 contents until they are entirely developed, when it disappears, and 

 the cell bursts. 



This encysting in reproduction of these organisms is constant 

 throughout a class which Grant has called Cystodia, and of the three 

 orders into which he has divided this class the Gregarinea, as he 

 calls them, is the first. It is also characteristic of animal life, and 

 may be added to the distinctions between plants and animals. It is 

 worthy of remark, as showing the border ground between animal and 

 vegetable life occupied by these cells, that M. Bary, of Prague, has 

 lately demonstrated that certain forms of fungi belonging to the 

 Mycetozoa are most probably referable to the Gregarince. These 

 fungi are, in Grant's words, "composed of aggregations of simple 

 cells, commonly referred to the vegetable kingdom." 



THE INFUSORIA. 



If a small quantity of hay, grass, or other vegetable or animal 

 matter be covered with water and allowed to stand for a few days 

 exposed to the atmosphere, and a drop be then placed under the 

 microscope, it will be found teeming with life. Hence the animal - 

 culae which may be produced by infusing organic matter are called 

 Infusoria. 



Their occurrence under the circumstances mentioned has given 

 rise to the theory now held with great pertinacity by M. Pouchet 

 and others of spontaneous or equivocal generation, which simply 

 means that the animal Protozoon may be produced and endowed 

 with life from the elements of dead organic matter. This doctrine, 

 however, which has been supported with great ability and careful 

 experiment by M. Pouchet, has been successfully combated by M. 

 Pasteur, who has satisfactorily proved the truth of the doctrine 

 hitherto held, that these organisms are produced from germs which 

 are always either floating in the atmosphere, or mingled with the 

 organic matter whose decay forms for them the " circumstances 

 necessary for their existence," at the time of which decay, and only 

 then, they produce the living creatures known as infusorial animal- 

 culae. The discussion between these inquirers, and a detail of the 

 experiments which each of them performed, have been published in 

 almost all the natural history magazines in this country during the 

 last four or five years, and it therefore is unnecessary for me to say 

 more upon the subject here. 



There are few objects more interesting, and to the student more 

 startling, than the examination of a drop of water from a stagnant 

 ditch, or a basin in which vegetable or animal matter, such as a 



