1893. EXPERIMENTAL EMBRYOLOGY. 303 
be due to an influence exerted on the cell-substance by the entrant 
spermatozoon. The delimitation is due, according to Fol, to the 
coagulation of a gelatinous substance between the envelope and the 
ovum. The Hertwigs and Herbst have shown that the hardening 
and the delimitation may be artificially evoked. 
Furthermore, Herr Herbst shook off the membrane from fer- 
tilised ova, and found that in a benzol mixture a second membrane 
was formed. He even succeeded occasionally in producing a second 
membrane in addition to the first in fertilised ova, and in producing 
two concentric membranes around unfertilised ova. 
The net result of this interesting little research is to show that: 
the conditions productive of the hardening and delimitation of the 
cortical layer of the ovum are to be found in the ovum itself, and 
are normally due to a stimulus associated with the entrance of the 
spermatozoon, but which may be artificially replaced by the 
stimulus of certain chemicals. 
Yet Weismann asks us to believe that the spermatozoon has no 
dynamic or other than merely quantitative influence on the ovum. 
There are many other recent researches which should be con- 
sidered in a full discussion of experimental embryology, but I shall 
refer only to two more. 
XI. Very striking is Boveri’s experiment, on which Weismann lays: 
great emphasis, and which one would like to see confirmed. Boveri 
artificially removed the nucleus from the ova of a species (A) of sea- 
urchin, and poured over them the spermatozoa of a related species 
(B). The ova, without any maternal nuclei, seemed to have their 
deficiencies supplied by the spermatozoa, and the larve which resulted 
had the characters of species B. 
To Weismann, this case supplies direct proof of the all-impor- 
tance of the nucleus in transmission. I must confess to some doubt 
as to the certainty with which specific characters of sea-urchins 
can be discriminated in the slarve. Moreover, hybridisation has 
sometimes strangely one-sided results, and from the non-appearance 
of any maternal characters, one cannot conclude with certainty 
that the cell-substance is only of nutrient importance in development. 
Furthermore, until definite proof to the contrary is forthcoming, it 
seems well to continue to believe that fertilisation is due to the 
spermatozoon and not merely to its chromatosomes. 
XII. Finally, we may refer to Heape’s remarkable experiment, in 
which, from an Angora doe rabbit (fertilised 32 hours previously by 
an Angora buck) he transferred two ova into the upper end of the 
Fallopian tube of a Belgian doe rabbit (fertilised three hours before 
by a Belgian buck). When the Belgian doe gave birth, four young 
were Belgian, two Angoras. 
A speculative discussion of some of the researches cited above will 
be found in Weismann’s *‘ Germ Plasm”’ (pp. 134-144), and to this the 
reader may in the meantime be referred. I prefer in this summary to 
leave the facts to speak for themselves. 
