SOME PROBLEMS OF REPRODUCTION. Ci, 
assume the behaviour of zygotes by developing as normal 
embryos.! 
6. ParacaMy comprises those cases where the fusion of 
sister-nuclei replaces the advent of a male nucleus. 
a. OopARAGAMY occurs in the Metazoa in this wise: after 
the first polar body is formed a second polar spindle is formed 
in the egg, as if to form a second polar body; but the nucleus 
corresponding with the second polar body moves back again 
to fuse with the nascent oosphere nucleus. This has been 
observed in Ascaris and Pterotrachea by Boveri, and 
in Asteracanthion by O. Hertwig. The fusion nucleus 
thus formed has the same number of chromatic elements as the 
normal zygote nucleus, double that of the gametogonium; and it 
is essentially different from the nuclei produced by mere fission 
during the whole cellular cycle since the last rejuvenescence. 
6. ApocyTIAL PARAGAMY occurs when the fusion of the 
nuclei of an apocytium wholly replaces the formation and 
union of gametes. This occurs in Saprolegniez and 
Derbesia. 
IX. GENERAL CONCLUSIONS. 
The following theses state concisely the results of our 
inquiry : 
1. Absolutely agamous forms exist in the group Monad- 
ine; in these rEsT is the only agent of rejuvenescence. 
2. Cuance or THE Mope or Lire is a frequent mode of 
rejuvenescence in apogamous and self-fertilising organisms. 
3. In the higher Monadinee and the Myxomycetes a 
plasmodium formation occurs,so that the cytoplasm is renewed 
by Prastocamy, and the nuclei wander from their original 
cytoplasts. 
1 The apogamic development of the plant of Pteris cretica from a group 
of cells in the prothallus, instead of from a fertilised oosphere, comes very 
close to this group of processes. I should note that apogamy is a purely 
negative word, implying solely the excision or loss of a sexual process 
from the life-history of an organism; but this may be effected and com- 
pensated in most divers ways, which might be classified if it were not 
rather outside the scope of tne present essay to do so. 
