1898.] MICROSCOPICAL JOURNAL. 185 



to us throug-h centuries of controversy were finally and for 

 all time dispelled by conclusive experiments upon animals 

 made during- the first half of this century. It was shown 

 that scab does not develope and cannot be produced with- 

 out the parasites. The complete life cycle of the mites 

 was studied and demonstrated from the eg-g-s to the adult 

 parasites. It was shown that mites are always the off- 

 spring of ancestors, the same as are the larg-er animals, 

 and it has in later years come to be admitted that there is 

 no such thing- known as spontaneous g-eneration of any 

 living- thing- under any circumstances. The demonstra- 

 tion was repeatedly made that the disease always de- 

 veloped if mites were taken from diseased sheep and 

 placed upon health}^ ones, and that diseases of the skin re- 

 sembling- scab are not contag-ious unless the mite is present. 

 The female mite lays about 15 to 24 eg-g-s on the skin, or 

 fastened to the woor near the skin; a six-leg-g-ed larva is 

 hatched ; these larvae cast their skin and become mature; 

 the mites pair and the females lay theii eg-g-s, after which 

 they die. The exact number of days required for each 

 stag-e varies somewhat, according- to the writing-s of dif- 

 ferent authors, a fact which is probably to be explained by 

 individual variation, and by the conditions under which 

 the observations and experiments were made. Thus Ger- 

 lach. in his well-known work (1857), estimates about four- 

 teen to fifteen days as the period required for a g-eneration 

 of mites from the time of pairing- to the maturity of the 

 next g-eneration. He divides this time as follows : Under 

 ordinary conditions the eg-g-s hatch in three to four days, 

 althoug-h two authors allow ten to eleven days for the eg-g- 

 stag-e ; three or four days after birth the six-leg-g-ed larvae 

 moult and the fourth pair of leg-s appears ; this fourth pair 

 is always present when the mites are two-thirds the size 

 of the adults ; when seven to eig-ht days old the mites are 

 mature and ready to pair ; several (three or four) days 

 are allowed for pairing-; another g-eneration of eg-g-s may 

 be laid fourteen to fifteen days after the laying- of the first 

 g-eneration of eg-g-s. Without g"oing- into all of the other 

 observations on these points, it may be remarked that the 



