DEVELOPMENT OF THE CHICK. 685 



During the course of the second day the embryo seems to sink 

 farther into the yolk, while both anteriorly and posteriorly double folds, 

 known respectively as the head and tail folds, rise up. In the course 

 of their development the embryo becomes completely " folded off" 

 from the yolk. At a slightly later stage, side folds also appear ; all the 

 folds now consist of a double layer of somatic mesoderm covered by 

 epiblast. The folds meet above the back of the embryo and coalesce. 

 The inner layer forms the true amnion, the outer the false amnion or 

 subzonal membrane. Into the space between the amniotic folds, a 

 diverticulum from the posterior region of the gut, the allantois, grows 

 out. 



Before the end of the first day, 

 blood vessels begin to be developed 

 in the extra- embryonic region of 

 the blastoderm. These form the 

 beginning of the vitelline vessels, 

 which are of great importance in the 

 early stages of development, and 

 have probably at first some respira- 

 tory importance. As development 

 proceeds, the allantois increases 

 greatly, and, fusing with the sub- 

 zonal membrane, approaches close 

 to the egg-shell. It has a large blood 

 supply, and functions as an organ of 

 respiration ; in addition it absorbs 

 the white of egg, thus serving as an 

 organ of nutrition ; it also receives 

 deposits of urates, thus functioning FlG 380. Diagrammatic section 

 in connection with excretion. of embry o within egg. After 



We have spoken of the "folding TOrmel 



rtJJ * 1 1 t f XVC1111C1. 



off" of the embryo ; as a result of _ __ , _ .. , 



this,.the embryo "is attached by a ^Jt'Sr/o; ^.^inTJ td 

 relatively narrow stalk to the large outer wall of the allantois ; am., 

 yolk-sac, over which the blastoderm amnion proper (the reference line 

 is now slowly spreading In this re- *} ^^f^L*! 

 spect the embryo strongly resembles membrane; /. is placed within the 

 that of the dogfish ; it differs from extra - embryonic body cavity into 

 the latter in the presence of the which the allantois grows, 

 overarching amniotic folds, and 



in the respiratory allantois, which functionally replaces the protruding 

 gills of the young dogfish. In the young tadpole the yolk lies heaped 

 up on the floor of the gut, and causes a certain amount of distortion. 

 In the chick, as in the embryo dogfish, the amount of yolk is so great 

 that it forms a hernia-like protrusion of the gut, and only at a very late 

 stage is the greatly reduced sac withdrawn into the body cavity, after 

 which the dermal and intestinal umbilical openings are closed. 



The chick embryo never exhibits any trace of gills, but the gill-clefts 

 perforate the pharynx. The embryonic organ of respiration is the 

 allantois, but that arrangement of aortic arches by means of which in 

 the tadpole blood is carried to the gills is repeated here. 



