32 Hybridisation of Amphibians [January, 
The following addual observations agree readily with the 
above hypothesis ; — The spermatozoa of R. fusca have, 
among all Amphibians, the thinnest anterior extremity, 
which terminates in a sharp point ; in consequence they 
fecundate almost all ova upon which they are allowed to 
adt, such as R. arvalis, R. esculenta, and Bufo communis . On 
the other hand, the thick-headed, blunt-pointed spermatozoa 
of R. arvalis and R. esculenta cannot penetrate into the ovum 
R' fusca. 1 he spermatozoon of Bufo communis, which 
has a thick head but a very sharp point, in some rare cases 
fecundates the ova of R. fusca. The spermatozoa of the 
tritons, having pointed heads, can also fertilise the ova of 
R. fusca. On the other hand, the thick-headed and blunt 
spermatozoa of R. arvalis and R. esculenta cannot fecundate 
any strange ovum, whilst reciprocal hybridisation between 
the two species is easy. 
In like harmony with the hypothesis the ova of R. escu- 
lenta are in the highest degree susceptible to the aftion of 
alien spermatozoa. They can be fecundated not alone by 
the male element of the frogs R. fusca and R. arvalis, but 
by that of the toads Bufo vulgaris and B. variabilis, and even 
B. calamita. The sperm of the tree frog ( Hyla arborea) was 
found also not quite inefficient. 
So fai, indeed, no Batrachian has been observed whose 
speimatozoa are less and whose ova are more adapted to 
hybridisation than those of R. esculenta. 
The same is the case with R. arvalis, also in accordance 
with the same hypothesis. 
Hen Pfliiger points out, however, that the experiments on 
hybridisation instituted by M. de ITsle, with R. fusca and 
R. agilis, whose spermatozoa are exactly similar in shape 
gave negative lesults, so that here other cases must come 
into play. 
The author’s experiments with toads agree also with the 
same hypothesis. Thus Bufo vulgaris, whose spermatozoa 
aie capable of adding upon the ova of R. fusca, is able to 
fecundate all ova which can be fertilised bv R. fusca,— i e 
R. esculenta, R. avails, Bufo variabilis, and B. calamita. B. 
variabilis, whose spermatozoa fecundate the ova of B. vul- 
garis should have spermatozoa of the same thickness, as is 
in ladt the case. The same holds good for the ova of the 
same species; they are more susceptible to hybridisation 
than those of R. fusca, but less so than those of R. esculenta 
and R arvalis. 
Hence it appears that an exadd knowledge of the 
relative magnitudes of the spermatozoa of different species 
