METHOD OF OBTAINING EMBRYOS. 271 



forceps the edge of the germinal area may be seized, and by gentle motion it may 

 be separated from the mass of yolk and also from the thin, whitish, overlying 

 membrane of the yolk, and at the same time from so much of the white of the egg 

 as may have been carried along. As one becomes more practised in these opera- 

 tions, it is not difficult to remove the germinal area without taking much yolk 

 along with it. 



The operation may be modified as follows : After the shell is opened the egg 

 may be tilted so as to allow the white to run off, and as it runs over the edge it is 

 snipped through with the scissors, and as much of the white removed as is possible 

 in this way. The whole egg is then submerged in the warm salt solution, an inci- 

 sion around the germinal area made as above, and the embryo floated off from 

 the yolk. 



Preservation of the Embryo. The next step, after the embryo has been re- 

 moved from the yolk and lies in the salt solution, is to put a glass slide in the salt 

 solution and carefully float the embryo and germinal area upon it, and then re- 

 move them together. The slide is now to be laid flat on the table and the germinal 

 area spread out carefully upon it. In this operation good results may often be 

 obtained by allowing a few drops of warm salt solution to fall upon the center of 

 the germinal area. The currents produced by the falling drops will be sufficient 

 to spread out the blastoderm in its natural form,* and at the same time to wash 

 away any superfluous yolk grains that may be adherent to the preparation. At 

 this stage the preparation should be examined by the student with a low power of 

 the microscope, as described below. To preserve the specimen, four or five drops 

 of Zenker's fluid are allowed to fall upon the specimen gently and quietly as it 

 lies upon the glass slide. The specimen is allowed to stand for about ten minutes 

 'and is then transferred to a dish containing a larger quantity of Zenker's fluid. 

 The transfer should be made by submerging one end of the slide in the dish and 

 floating the specimen off. In from two to four hours the hardening of the speci- 

 mens will be completed. They must then be washed thoroughly by decanting 

 off the Zenker's fluid and replacing it with water, and this water must itself be 

 replaced several times during the next twenty-four hours. Further treatment 

 of the specimen is as described on page 363. 



The Making of Serial Sections. Specimens are best colored with alum cochi- 

 neal in toto. They are then imbedded in paraffin and cut into series. The most 

 useful sections are those which are transverse to the axis of the spinal cord. They 

 should not exceed 10 // in thickness. 



* The student will observe that the fresh blastoderm is very easily distorted. 



