Alternation of Generations and Ontogeny. 213 
described as distinct species. If we take Polystomella we find two 
forms, the microspheric and megalospheric living under the same 
conditions in sea water. The protoplasm of the microspheric form 
passes out of the cyst and breaks up into a number of brood-cells, 
each of which forms a megalospheric form; the microspheric is thus 
the asexual form. The megalospheric form is the sexual one 
and produces gametes which fuse and give origin to the microspheric 
form 1 . We have then an alternation of sexual and asexual genera¬ 
tions similar to that of ferns, but both generations start their life 
in a free condition in the water and yet they are somatically distinct. 
The explanation of the differences between the generations must 
clearly be sought, not in external conditions, but in the obscurities 
of the protoplasmic mechanism. 
The case of other animals which show complicated life-cycles, 
parts of which are lived under very different conditions, appear to 
give no support to the hypothesis. In the Protozoa where there is 
no soma the different behaviour of the different cells must be largely 
controlled by internal stimuli related to the position of the cells in 
the life-cycle. When a mosquito sucks the blood of a human being 
infected with malaria, it may draw into its stomach various stages 
of the parasite along with the blood. Young amoebulse, full-grown 
schizonts, rosettes and crescents, all alike may be swallowed but 
with different results. Only the crescents are able to resist the 
action of the digestive juices and so to develop further. Clearly we 
have a differentiation among the various free cells which make up the 
life-cycle. Only the crescents can develop in the mosquito and 
they cannot develop further in man. Later on another type of cell is 
produced, the sporozoite, which cannot develop further in the 
mosquito but can in man. If a simple form like the malarial parasite 
can produce free cells with different tendencies we may expect the 
same in ferns. To take another example, the liver-fluke, Distomuin 
hepaticum, has a complicated life-cycle in which we have three types 
of free structures, sporocyst, redia and cercaria. They show 
considerable differences but all are built up on a common plan, 
although the first develops from an egg in water while the other 
two start their development within the mother-tissues and inside a 
molluscan host. The two latter, however, differ considerably, 
although produced under fairly similar conditions. These com¬ 
plicated life-cycles in animals would seem only to be explained by 
the fact that cyclical correlation plays a very large part. 
1 The megalospheric form has been seen in some genera to form 
similar megalospheric forms, this would appear to correspond 
to gametophytic budding. 
