MONADS 761 



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 two flagella ; 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 amo3boid condition, in which the 

 creature not only moves, but also feeds, like an Amveba (devouring all 

 the living and dead Bacteria in its neighbourhood), occurs previously 

 to ' conjugation ; ' and this takes place between two of the amoeboid 

 forms, which begin to blend into each other 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 flagella are lost, and a 

 spherical distended sac is then formed, which, in a few r hours more 

 without any violent splitting or breaking up, sets free innumerable 

 masses of reproductive particles. These under a magnifying power 

 of 2,500 diameters can be just recognised as oval granules, which 

 rapidly develop themselves into the likeness of their parents, and 

 in their turn multiply by duplicative fission, thus completing the 

 ' genetic ' cycle. 



One of the most important researches thus ably prosecuted by 

 Messrs. Dallinger and Drysdale has reference to the temperatures 

 respectively endurable by the adult or developed forms of these 

 monads, and by their reproductive germs. A large number of experi- 

 ments 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 tempera- 

 ture 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 highest interest in its bearing on the 

 question of * spontaneous generation,' or abiogenesis ; since it shows 

 that germs capable of surviving desiccation may be everywhere diffused 

 through the air, and may, on account of their extreme minuteness 

 (as they certainly do not exceed ^ ooVo^th 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 



Beyond these facts others of some importance, as well as a new 



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

 in their researches will be found in Monthly Micros. Jon rn. vol. xi. 1874, p. 97 ; ibid. 

 vol. xv. 1876, p. 165 ; and Proceed. Boy. Soc. vol. xxvii. 1878, p. 343. 



