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 hive 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. Ha 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 conditioas 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 ihe 

 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 miny 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 Bo veri's experiment, on which Weismann lays 

 greit 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 larvae which res ulted 

 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 'larvae. Moreover, hybridisation has 

 sometiines strangely one-sided results, and from the non-appearance 

 of any maternal characters, one cannot conclude with certainly 

 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. 



Xn. 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. 



