a Lesson in Biogenesis. 57 



wriggling motion, when the flagella were discovered, and they 

 very soon moved freely over the field. Their increase in size was 

 now rapid, arising probably from the greater amount of pabulum 

 secured by motion, and certainly aided by a considerable vacuola- 

 tion, as shown in Fig. 6, drawn at the expiration of ten hours ; and 

 at the end of a little less than twelve hours, the normal parent form 

 was taken, as seen in Fig. 7, which in about forty minutes after had 

 begun to divide by fission ; one that was watched from the begin- 

 ning is drawn at Fig. 8, when in the middle of this process. 



Thus the entire life cycle of this form is seen. When mature, 

 it multiplies by fission for a period extending over from two to 

 eight days. It then becomes peculiarly amoeboid ; two individuals 

 coalesce, slowly increase in size, and become a tightly-distended 

 cyst. The cyst bursts, and incalculable hosts of immeasurably 

 small sporules are poured out, as if in a viscid fluid, and densely 

 packed ; these are scattered, slowly enlarge, acquii-e flagella, be- 

 come active, attain rapidly the parent form, and once more increase 

 by fission. 



We were now desirous of ascertaining to what extent the 

 mature form and the sporule were pervious to the action of heat. 

 There was considerable difi&culty in this, since it was necessary to 

 know that the sporules were in the heated drop, and this could only 

 be positively asserted of that which was in the field of the instru- 

 ment. An ordinary slide, containing adult forms and sporules 

 covered in the ordinary way, was allowed to evaporate slowly, in 

 seven instances, and placed in a dry heat which was raised to 121° C. 

 It was then slowly cooled, and distilled water allowed to insert 

 itself by capillary attraction. On examination all the adult forms 

 were absolutely destroyed, and no spore could be definitely identified. 

 But after being kept moist in the growing stage for some hours 

 and watched with the J^, gelatinous points were seen in two out of 

 the seven cases, which were recognized as exactly like an early stage 

 of the developing sporule, which were watched until they had 

 reached the small flagellate state shown in Fig. 5, Plate XXVI. 

 The remaining five were barren of result. 



In boiling, the difiiculties were still greater, from the uncertainty 

 beforehand of the presence of sporules in a sufficiently large drop 

 of the infusion casually taken. After, however, a considerable 

 number of experiments, we find that a temperature of 66"' C, given 

 to the infusion, destroys all adult forms ; but we have found young 

 monads appear and develop in an infusion which has been raised to 

 127° C. ; suggesting that the sporule is uninjured in a temperature 

 considerably above that which is wholly destructive of the adult. 



We are aware of the valuable generalization of Cohn and Hor- 

 wath, founded upon accurate experiments, that all living things are 

 destroyed at a temperature of 62° C. when equal difi'usion of heat 



