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 slide, remove enough 

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

 of tissue paper with an opening large enough to leave the germinal disc exposed. Add a 

 few drops of fixative and float embryo into a covered dish. Following the routine 

 technical procedure, embryos are stained and sectioned serially or mounted entire. 



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 ; ' later ad to indicate 'toward the side ; ' and mesad, 'toward the 

 median plane.' 



EMBRYOS OF ABOUT TWENTY HOURS' INCUBATION 



The events of cleavage and the formation of the three primary germ 

 layers in birds have been described in the preceding chapter. The appear- 

 ance on the disc-like blastoderm (Fig. 3) of the primitive streak and groove 

 (Fig. 23), and its cranial extension, the head process (Fig. 25), has likewise 

 received brief treatment (pp. 32^33). 



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

 streak is formed as a li near opacity near er the posterior border of the germinal 

 disc. Over a somewhat pear-shaped clear area the yolk has been dis- 

 solved away from the overlying entoderm. This area, from its appear- 

 ance, is termed the area pellucida. It is surrounded by the darker and 

 more granular area opaca, which constitutes the remainder of the fairly 

 sharply limited blastoderm. Whether or not the primitive streak repre- 

 sents the fused lips 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 extends at first more rapidly caudal 

 and lateral to the primitive streak. However, there is soon an axial 

 growth forward from the thickened primitive knot (Figs. 23 and 30). This 

 is the head process, or notochordal plate. It temporarily fuses with the 

 entoderm (p. 33) and is continuous laterally with expanding mesodermal 

 sheets (Figs. 25 and 31). 



37 



