CHAPTER III 

 THE STUDY OF CHICK EMBRYOS 



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

 indicate "towards the back" or "towards the belly"; cephalad and cranially to 

 denote "headwards," caudad to denote "tailwards," and later ad when the loca- 

 tion is at the side. As there is no single word in English to express the primi- 

 tive cellular germ of a structure, the German word anlage has been adopted by 

 embryologists and will be used here. 



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 area, 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 opening large enough to leave the germinal area 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 acid Haematoxylin (Conklin) 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 ac- 

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



EMBRYO OF SEVEN SEGMENTS (TWENTY-FIVE HOURS' INCUBATION) 

 In this embryo (Fig. 31) there is a prominent network of blood-vessels and 

 blood-cells in the caudal portion of the area opaca. In its cranial portion isolated 

 groups of blood and blood-vessel forming cells are seen as blood-islands. To- 

 gether, they constitute the angioblast from which arises the blood vascular system. 

 The area pellucida has the form of the sole of a shoe with broad toe directed for- 

 ward. The head-fold has become cylindrical and the head of the embryo is free 

 for a short distance from the germinal area. The mesoderm extends on each side 

 beyond the head leaving a median clear space, the proamniotic area. The en- 

 toderm is carried forward in the head-fold as the fore-gut, from which later arise 

 the pharynx, esophagus, stomach and a portion of the small intestine. The 

 opening into the fore-gut faces caudad and is the fovea cardiaca. The way in 

 which the entoderm is folded up from the germinal disc and forward into the 

 head is seen well in a longitudinal section of an older embryo (Fig. 32). The 



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