EMBRYOLOGY OF THE CHICK 321 



which it ultimately incloses, the margins of the mtellicle so formed 

 contracting and uniting at the side opposite the embryo at a sort_pf_ 

 cicatrix, to which the last part of the abdominal yolk adheres. The 

 vitellicle is richly vascular, and the surface next to the yolk is aug- 

 mented by rugae. 



The fowl's egg, at about the fortieth hour, shows the buds from 

 which the limbs are developed. A vesicle is seen to protrude near 

 the anal end of the intestine, which, rapidly expanding, spreads over 

 the embryo, acquiring a close adhesion to the amnion, but remaining 

 distinct from the vitellicle, over which it spreads. It finally en- 

 closes the albumen and interposes itself between the latter and the 

 lining membrane of the shell. Umbilical vessels are associated with 

 this membrane. Hunter called this membrane the allantois from 

 its containing urine, and Owen states that the sac which surrounds 

 the albumen acts as the chorion or placenta; for it is most probable 

 that from this surface the albumen is absorbed and the chick sup- 

 ported on its developmental food. The external part of the sac 

 apparently acts as lungs as it comes into contact with the shell of 

 the egg through pores of which there is an exchange of air. Oxygen 

 is consumed and carbon dioxide is given off. The blood in the 

 vessels of this membrane is in color more like arterial blood and that 

 in the interior more like venous blood. 



The embryonic mass of the incubating egg always floats to the top 

 side. As the embryo grows it turns upon its left side, exhibiting a 

 profile view; it then indents the yolk, and finally almost divides it 

 into two parts. 



The peripheral layers of cells rise from the margin of the germ 

 mass, and extend and contract toward the opposite pole. This 

 tract of germ substance is the primitive streak. Along the median 

 line it next forms a furrow, which stops short of the ends of the streak. 

 This streak terminates opposite the point from which the germ 

 begins, and swells into the head. The median furrow expands upon 

 it. The cephalic borders are next united by a thin layer of epithelial 

 cells above the furrow, converting it into a cavity, or ventricle. The 

 myelonal furrow is similarly convered by a layer, uniting the lateral 

 columns. The embryonal trace becomes longer, narrower, and 

 bends round the vitellus. A layer of epithelial cells forms a net- 

 work over the whole dorsal surface of the embryo. Oblique striae 

 appear in the broadening germ mass radiating from the primitive 

 streak. These indicate divisional segments. These beginnings 

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