368 Dr. H. M. Woodcock and Mr. G. Lapage. 



natural, as in parasitism, seems to diminish the necessity for syngamy " ; from 

 this stage it is only a step further to find the capacity for syngamy lost. 



Bearing on the Case of the Trypanosomas. — It appears to us that this remark- 

 able experimental fact has an important bearing on the question of the 

 trypanosomes, and may afford an explanation of why conjugation (or syngamy); 

 in these parasites, though assiduously sought, has not been observed in any 

 authenticated instance. Probably it never does occur, because these forms 

 have lost the power to conjugate. Just as, in the case of the above-discussed 

 strain of Helkesimastix, the rapid, successive transferences to fresh, non- 

 toxic medium at first removed the necessity for conjugation (and encystment), 

 and then have led to the loss of the ability to undergo syngamy (and, in this 

 case, to form cysts), so a similar development has very likely occurred in the 

 trypanosomes and related forms. As is obvious, the conditions of life in these 

 parasites are readily comparable to those above described. The trypano- 

 somes live in a rich nutritive medium, namely blood, in the vertebrate, or 

 blood in various stages of digestion, in the invertebrate host ; and, as is Avell 

 known, rapid multiplication in both hosts is usually found. In the inverte- 

 brate we get, more or less frequently, replenishment, i.e. a fresh supply of 

 the " constant " medium (namely blood), together with removal of toxic 

 products. In the blood of the vertebrate, where there is at first abundance 

 •of medium, but not any " fresh " supply, it is very interesting to note that, 

 in the case at all events of many lethal trypanosomes, which, as Minchin has 

 pointed out, are not yet completely adapted to their hosts, we get the pro- 

 duction eventually — after active multiplication has gone on for some time — 

 of the well known " involution " forms. These, at any rate in our opinion, 

 as in that of many French workers, simply die off after a time, if left to 

 themselves ; but they are capable of being " rejuvenated " and of again 

 multiplying actively if passed into a fresh host. And a similar state of 

 affairs has been met with by one of us in cultures of avian trypanosomes. 

 Now it seems to us that these two cases present a very close parallel to what 

 we have observed in Helkesimastix. It may be pointed out, also, that the 

 non-conjugating strain of Helkesimastix is the result of an abruptly 

 originating and more or less artificial intensive culture, whereas in the case 

 of trypanosomes and allied parasitic flagellates the corresponding conditions 

 have operated naturally and over a long duration of time. 



On the supposition that syngamy has now been entirely lost in the hseino- 

 flagellates, the explanation which we here put forward is much more probable, 

 we think, than the idea, which has also been suggested, that the stimulus 

 afforded by the change of hosts is accountable for the loss of this process 

 in the trypanosomes. We have always considered that there are two or 



