CHAPTER III 

 THE STUDY OF CHICK EMBRYOS 



Chick embryos may be studied whole and most of the structures identified up to the 

 end of the second day. The eggs should be opened in normal saline solution at 40 C. With 

 scissors cut around the germinal disc, float the embryo off the yolk, and remove the vitelline 

 membrane. Then float the embryo dorsal side up on a glass sUde, remove enough of the saline 

 solution to straighten wrinkles, and carefully place over the embryo a circle of tissue paper 

 with opening large enough to leave the germinal disc exposed. Add a few drops of fixative 

 (5 per cent, nitric acid gives good fixation) and float embryo into a covered dish. After fix- 

 ing and hardening, stain in Conklin's acid hematoxylin or in acid carmine. Extract sur- 

 plus stain, clear, and mount on slide supporting cover-slip to prevent crushing the embryo. 

 Acid hsematoxylin gives the best results for embryos of the first two days. For a detailed 

 account of embryological technique see Lee's "Microtomist's Vade Mecum." 



In the following descriptions we shall use the terms dorsad and ventrad to indicate 

 "toward the back" or "toward the belly"; cephalad and craniad to denote "headward"; 

 caudad to denote "tailward"; laterad to indicate "toward the side"; and mesad, "toward the 

 middle line." 



EMBRYOS OF ABOUT TWENTY HOURS' INCUBATION 



The events of cleavage and the formation of the primary germ layers in 

 birds have been described in an earher chapter. The appearance on the disc- 

 like blastoderm (Fig. 3) of the primitive streak and groove (Fig. 23), and of its 

 cranial extension, the head process (Fig. 25), has likewise received brief treatment 

 (p. 31). 



In a chick embryo of twenty hours' incubation (Fig. 30) the primitive streak 

 is formed as a linear opacity near the posterior border of the germinal disc. 

 Over a somewhat pear-shaped clear area the yolk has been dissolved'away from 

 the overlying entoderm. This area, from its appearance, is termed the area 

 pellucida. It is surrounded by the darker and more granular area opaca. Whether 

 or not the primitive streak represents the fused Hps of the blastopore, it is certain 

 that it represents the point of origin for the middle germ layer, the extent of 

 which is indicated by the shaded area of Fig. 30. It also indicates the future 

 longitudinal axis of the embryo. The mesoderm extend^; a^fi rst more ra pidly 

 cau d a l to t he p rim i tive_s treak, at the cranial end of which appea rs_a^haded 

 thickenirig,_the ^mi!i;z)e knot or node (of Hensen). From the primitive knot it 

 grows cranially, forming along the midline a thicker layer of tissue, the noto- 

 36 



