Observations on the Life-Cycle of Helkesimastix fsecicola. 365 



We have not actually watched a motionless, rounded zygote through its 

 encystment period, hut we noted the position of the above individual and of 

 others at the same time late in the evening, and on looking at them next 

 morning we found they had become the very characteristic " shrinkage " cysts 

 (figs. 68 and 69). The cyst- wall has been formed and the protoplasm has 

 continued to contract, so that a space is left at one side between the body and 

 the wall. We are inclined to think it is a space containing no liquid, because 

 it is always very clear. The space appears of different size, according to the 

 age of the cyst (figs. 68 and 69). When small the space is spherical, but 

 when it attains its maximum size it is lens-shaped. A curious fact is that 

 after the cyst ha? shown this condition of shrinkage for some time — the 

 period may vary from two days or less up to longer— the space disappears 

 entirely and the cyst becomes a permanent cyst, as described at the com- 

 mencement of this account. This finishes the life-cycle. 



Biological Notes. — As, in a normal culture, the great majority of the 

 flagellates form cysts, one could not have a more convincing and readily 

 obtained demonstration of the regular occurrence of syngamy in the life- 

 cycle of these simple dung-flagellates. Although all the above observations 

 on Helkesimastix have been described either from plate-cultures or from 

 observation-preparations, this is only because, on account of the more rapid 

 multiplication and development under those conditions, the individuals are 

 very much more abundant than in a simple, moistened dung-culture, and 

 therefore the different stages are more readily found. We have not the 

 slightest doubt that the life-cycle is exactly similar under the more natural 

 conditions, the only difference being that it is probably slower, i.e. a longer 

 period may elapse before the whole life- cycle is completed. For instance, 

 it is usually three or four days before this flagellate is observed in the active 

 condition in a dilute dung-culture, and it may persist in the active condition 

 for a week or more before forming cysts. The reason for this is, we consider, 

 because the watery medium is not nearly so rich in nourishment as the beef- 

 extract medium. The bacterial development is not nearly so great as in the 

 latter case, nor is the multiplication of the flagellates so abundant. Therefore, 

 on the one hand, a somewhat longer time most probably elapses before the 

 cyst-wall is dissolved ; and, on the other hand, the medium does not so soon 

 become excessively full of the " toxic " products, or whatever chemical 

 substances induce the cessation of multiplication and the tendency to 

 conjugation, whether formed from the flagellates, or the bacteria, or from 

 both. As, however, we are at present engaged in making a full study of 

 this interesting flagellate from a biological standpoint we will not further 

 discuss these questions at present. 



