176 STUDY OF YOUNG CHICK EMBRYOS. 



possible in this way. The whole egg is then submerged in the warm salt solution, 

 an incision 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 removed 

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

 tion and carefully float the embryo and germinal area upon it, and then remove 

 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 378. 



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

 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 10/1 in thickness. 



Embryo Chick with Eight Segments. (About twenty-eight hours' incubation.) 



The following description is almost equally applicable to embryos with six or 

 ten segments. 



Examination in the Fresh State. The embryo when first removed from the 

 yolk should be placed in a staining-dish with a small quantity of normal salt solu- 

 tion and examined with a low power of the microscope as a transparent object. 

 The specimen as a whole has a grayish or brownish gray tint. Most of the germi- 

 nal area is dark, the transmission of light being stopped by the numerous yolk- 

 grains contained in the entodermal cells (compare page 64). In the center of the 

 germinal area the transparent area pellucida is very conspicuous, and has an edge 

 which is quite sharply defined, more so than after the specimen has been preserved. 

 It is shaped somewhat like an elongated pear, the broad end of which surrounds 



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



