MICROSCOPIC FOKMS OF ANIMAL LIFE. 31 



fission, which is preceded by a change to a semi-amoeboid state. This 

 gives place to a more regular pear-like form, the four flagella issuing 

 from the large end; and the fission commences at their base, two pairs 

 being separated by the cleavage-plane. The nucleus also undergoes 

 cleavage, and its two halves are carried apart by the backward extension of 

 the cleavage. The two half-bodies at last remain connected only by their 

 hinder prolongations, which speedily give way, and set them free. Each, 

 however, has, as yet, only twofiagella; but these speedily fix themselves by 

 their free extremities, undergo a rapid vibratory movement, and in the 

 course of about two minutes split themselves from end to end. A still more 

 complete change* into the amoeboid condition, in which the creature not 

 only moves, but also feeds, like an Amoeba (devouring all the living and 

 dead Bacteria in its neighborhood), occurs previously to ' conjugation;' 

 and this takes place between two of the amoeboid forms, which begin to 

 blend into one another almost immediately upon coming into contact. 

 The conjugated bodies, however, swim freely about for a time, the two 

 sets of flagella apparently acting in concert. But by the end of about 

 eighteen hours, the fusion of the bodies and nuclei is complete, the fla- 

 gella are retracted, and a spherical distended sac is then formed, which, 

 in a few hours more, without any violent splitting or breaking up, sets 

 free innumerable masses of reproductive particles. These, under a mag- 

 nifying power of 2,500 diameters, can be just recognized as oval granules, 

 which rapidly develop themselves into the likeness of their parents, and 

 in their return multiply by duplicative fission, thus completing the 

 ' genetic ' cycle. 



422. One of the most important researches thus ably prosecuted by 

 Messrs. Dallinger and Drysdale, has reference to the Temperatures re- 

 spectively endurable by the adult or developed forms of these Monads, 

 and by their reproductive germs. A large number of experiments upon 

 the several *forms now described, indubitably led to the conclusion that 

 all the adult forms, as well as all those which had reached a stage of 

 development in which they can be distinguished from the reproductive 

 granules, are utterly destroyed by a temperature of 150 Fahr. But, on 

 the other hand, the reproductive granules emitted from the cysts that 

 originate in ' conjugation ' were found capable of sustaining a fluid heat 

 of 220, and a dry heat of about 30 more, those of the Cercomonad 

 surviving exposure to a dry heat of 300 Fahr. This is a fact of the high- 

 est interest in its bearing on the question of ' spontaneous generation ' 

 or Abiogenesis; since it shows (1) that germs capable of surviving desic- 

 cation may be everywhere diffused through the air, and may, on account 

 of their extreme minuteness (as they certainly do not exceed 1-200, 000th 

 of an inch in diameter), altogether escape the most careful scrutiny and 

 the most thorough cleansing processes; while (2) their extraordinary 

 power of resisting heat will prevent these germs from, being killed either 

 by boiling, or by dry-heating up to even 300 Fahr. 1 



423. The structural resemblance of these simple Flagellate Infusoria 

 to the ' Monads ' of Volvox and its allies ( 237), is so close that no other 

 than physiological reasons can be assigned for separating them. Whilst 

 the VolvocinecB grow and multiply under conditions which seem to jus- 

 tify our regarding them as members of the Vegetable Kingdom ( 220), 



1 Descriptions of the special apparatus used by Messrs. Dallinger and Drysdale 

 in their researches will be found in "Monthly Micr. Journ.," Vol. xi. (1874), p. 

 97; ibid., Vol. xv. (1876), p. 165; and "Proceed. Roy. Soc.," Vol. xxrii. (1878), 

 p. 343. 



