190 LECTURE VIII. 



liar properties for lengthened periods, nay, that these 

 might cling to it for many years. Of course it was im- 

 possible at the same time to entertain the opinion, that 

 the constituents of the blood were of a perishable nature, 

 and that new elements were added to it, to replace the old 

 ones. For the durability of a part as such presupposes 

 either that all its individual particles are durable, or that 

 these individual particles are continually producing fresh 

 ones within the part which bear impressed upon them all 

 the peculiarities of the old ones. In the case of the 

 blood, therefore, one would have to assume that its con- 

 stituents really did subsist for years, and could for years 

 present the same changes, or one would have to imagine 

 that the blood transmitted something from one particle 

 to another, and that from a parent blood-cell to its pro- 

 geny something hereditary was handed down. Of these 

 possibilities the former has, I believe, at the present time 

 been pretty generally discarded. No one, I think, now 

 imagines that the individual constituents of the blood last 

 on for years. On the other hand, the possibility that the 

 corpuscles of the blood are renewed by propagation, 

 and that certain peculiarities which are introduced into 

 the blood at a certain time, are transmitted from cor- 

 puscle to corpuscle, cannot straightway be rejected. 

 But the only phenomena pointing to such a propagation 

 of the blood, concerning which we possess any positive 

 information, belong to an early period of embryonic life. 

 There it appears from observations which were only the 

 other day again confirmed by Remak, the existing blood- 

 corpuscles undergo direct division, the process being that, 

 in a corpuscle which during the early stages of its deve- 

 lopment had displayed itself as a nucleated cell, first of 

 all a partition of the nucleus takes place (Fig. 51, c) ; 

 and that then the whole cell becomes constricted in the 

 middle, and gradually is really seen to pass into a state 



