1884.] 
3i 
and the Principles of Reproduction. 
As regards hybridisation the results obtained may be laid 
down in the following proportions : — 
1. Pfliiger and W. J. Smith have obtained three living 
frogs sprung on the mother’s side from R. arvalis, the 
father being R. fusca. Herr Born has been much 
more widely successful in this respect, and has ob- 
tained many such specimens. 
2. Perfectly vital semen and perfectly vital ova, capable 
of giving rise to perfect normal ova, possess only for 
a brief time the power of producing hybrids. This 
stage is that of the most intense heat. After it is 
past the ova still react normally with the semen of 
their own species. This fadt demonstrates that the 
unfecundated germ and the sperm-cells before fecun- 
dation are subjedt to continuous internal change. 
This rule holds good most sharply and distinctly for the 
ova, whilst the sperm is still capable of effecting a hybridised 
fecundation for some time before and after the most intense 
stage of heat. 
3. Herr Pfliiger succeeds in demonstrating that hybrid 
fecundation can be perfectly reciprocal. The sperm 
of R. esculenta fecundates the ova of R. arvalis as 
energetically as the sperm of R. arvalis fertilises the 
ova of R. esculenta. 
4. More generally, however, hybridisation is one-sided. 
The sperm of R. fusca fecundates energetically the 
ova of R. esculenta , but the sperm of the latter has 
no action upon the ova of the former. The sperm of 
R. fusca does not fecundate the ova of the Triton 
(newt), whilst the sperm of the latter fertilises the ova 
of R. fusca. 
This one-sidedness, if we may so call it, Prof. Pfliiger ex- 
plains by means of the following facts : — 
a. Those spermatozoa are most capable of effecting hybrid 
fecundation which have their anterior part thin and 
pointed. 
b. The ova of species whose spermatozoa have a thick 
anterior extremity are most liable to hybrid fecun- 
dation. 
We have thus a simple mechanical explanation of the 
one-sidedness of hybridisation, the micropyle being exactly 
wide enough to allow the spermatozoa of the same species 
to penetrate into the interior of the ovum. 
