166 EMBRYOLOGY. 



small quantity of one per cent, solution of bichromate of pot- 

 ash, which, after one or two days, is replaced by from one-sixth 

 to one-tenth per cent, chromic acid solution. In two or three 

 days more, the segment of yolk which bears the blastoderm is 

 cut off and transferred to spirit ; the vitelline membrane is 

 then carefully removed. Afterwards the object, which may or 

 may not be stained, is treated with absolute alcohol, embedded, 

 and employed for sections in the manner above described. 

 This method may be employed during the first twenty-four 

 hours of incubation. At a later period, or at all events after 

 thirty-six hours, the egg must be treated in the following 

 manner : 



After the yolk is freed from albumen, the vitelline mem- 

 brane is snipped with scissors at a point in its periphery as 

 far removed from the blastoderm as possible; part of the yolk 

 flows out through the opening, while the blastoderm adhering 

 to the vitelline membrane remains in position. The vitelline 

 membrane is then cut around the blastoderm, the circular 

 piece not only including the blastoderm, but the vitelline mem- 

 brane over it, together with a portion of yolk under it. This 

 is placed in a small flat watch-glass, which is held by forceps, 

 and is brought into a glass capsule containing a very weak 

 solution of bichromate of potash. After from ten to fifteen 

 minutes, the edge of the vitelline membrane is seized by for- 

 ceps, and gently swaj'ecl to and fro in the liquid till that mem- 

 brane is loosened and removed. The blastoderm, with the 

 yolk adhering to its area pellucida, is thus completely iso- 

 lated. 



In the superficial portion of the germ disks thus isolated, 

 especially those of the early part of the second day of incuba- 

 tion (provided that they are normally developed as is usually 

 the case in spring and summer), the primitive streak, the rudi- 

 ments of the central nervous system, of the chorda dorsalis, of 

 the proto vertebra;, of the heart and great vessels, of the eyes, 

 of the auditory vesicles, and of the olfactory pits, may be ob- 

 served. For this purpose, the blastoderm is floated from the 

 watch-glass on to an object-glass, and examined with a low 

 power. For studying the first vessels it is necessary to use 

 higher powers. The object, in solution of bichromate of potash, 

 or in a mixture of this and glycerin, should be surrounded b}' 

 a ring of zinc foil, wax mass, putty, or sealing wax, and 

 covered. The whole germ disk of the second day of incuba- 

 tion, which is very suitable for the demonstration superficially 

 of the rudiments of the organs just named, may be preserved 

 for a considerable time, if the wall of sealing-wax surrounding 

 the blastoderm is high enough. The mixture consists of one 

 part of one-sixth per cent, chromic acid, two parts of one-half 



