DEVELOPMENT OF CHlCK 



767 



at first diverge posteriorly on either side of the primitive streak, but 

 as the union travels backwards, this is included in the medullary canal, 

 and so disappears. 



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 ectoderm. 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 am- 

 niotic 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 

 respiratory significance. As de- 

 velopment proceeds, the allantois 

 increases greatly, and, fusing with 

 the subzonal membrane, ap- 

 proaches close to the egg-shell. 

 It has a large blood -supply, and 

 functions as an organ of respiration , 

 of egg, thus serving as an organ of nutrition ; it also receives deposits 

 of urates, thus functioning in connection with excretion. 



We have spoken of the " folding off " of the embryo ; as a result of 

 this, the embryo is attached by a relatively narrow stalk to the large 

 yolk-sac, over which the blastoderm is now slowly spreading. In this 

 respect the embryo strongly resembles that of the dogfish ; it differs 

 from the latter iri the presence of the overarching amniotic folds, and 



Fig. 459. — Diagrammatic section 

 of egg. — After Allen Thomson. 



g.v., Position of germinal vesicle; 

 a.c, air-chamber ; Y., yolk (al- 

 ; temate layers of " yellow " and 

 " white ") ; ch., chalaza. 



in addition it absorbs the white 



a.o., area 

 M., meso- 



3. Diagrammatic surface view, a.p., Area pellucida ; 



opaca ; n.p., neural groove ; p.s., primitive streak ; 

 derm (or mesoblast) spreading over yolk. 



4. Diagrammatic surface view at later stage, a.p., Area pellucida ; 



a.o., area opaca ; m.s., mesoderm segments ; p.s., primitive 

 streak. The dark border shows the spreading of the mesoderm 

 over the yolk. 



5. Cross-section, s.c, Spinal cord ; s.g., rudiment of spinal gangha ; 



N., notochord ; m.p., mesoderm plates; A., aorta; Am., 

 amnion fold ; c, coelom or pleuro-peritoneal cavity. 



6. Embryo. Cb., Cerebellum ; F., ear ; H., heart ; /./., fore-limb ; 



h.l., hind-limb; y.s., stalk of cut-off yolk-sac; Al., allantois; 

 £., eye ; C, cerebrum. On the dorsal surface the mesodermal 

 somites are indicated. 



