250 WATASE. [Vol. IV. 



acid, and osmic acid, in the proportion recommended by the 

 Hertwigs for their well-known macerating fluid. The propor- 

 tion of the osmic acid may be reduced, or may be altogether 

 excluded from the reagent, as it affects the eyes of the observer 

 while he is closely watching the effect of the reagent upon the 

 tgg and teasing away the blastoderm with needles. I found it 

 most satisfactory to remove the ovum from the action of the 

 reagent as soon as the translucent protoplasmic germ-disc turns 

 whitish opaque, which takes place very quickly, and complete 

 the operation of separating the blastoderm in dilute glycerine. 



The blastoderm thus separated may afterwards be stained with 

 dilute Schneider's acetic carmine, or with Erlich's hasmatoxylin, 

 and then mounted in glycerine ; or may be left unstained, as 

 most things can be satisfactorily made out without any staining, 

 and by avoiding this unnecessary process of technique in this 

 case, the risk of dislocating or injuring the delicate blastoderm 

 in other ways is thus greatly lessened — the risk all the more to 

 be avoided as the blastoderm is not firmly fixed to the slide, 

 and a little movement of the cover-glass is often enough to 

 destroy the whole specimen. 



Treatment of living eggs with Perenyi's fluid for a few seconds 

 is excellent for the surface study of cleavage. It turns the 

 protoplasmic portion of the ovum opaque yellow, and brings 

 out the cleavage furrows distinctly, while the rest of the tgg 

 remains translucent as before. 



II. 



It was a disputed question at the time when the cell-theory 

 was first promulgated, how much of the animal ovum is to be 

 homologized with Schwann's scheme of an organized cell — 

 whether the germinal vesicle alone is to be taken as such, or 

 the whole substance of the ovum, including both the cytoplasm 

 and the germinal vesicle. 



This failure to recognize the true cellular nature. of the ovum 

 and of the cleavage segments, coupled with the false doctrine of 

 the Cytoblastema of Schwann, stood in the way of a true under- 

 standing of the cleavage process, and obscured the significance 

 of the cell-theory itself. 



Our understanding of the cell-theory was made possible {i) by 



