DE VRLOPMENT OF THE CHICK. 683 



with which it is already invested ; on the other hand, segmentation goes 

 on rapidly in the formative area. 



The fully formed and laid egg is surrounded by a firm porous shell of 

 carbonate of lime, and beneath this there is a double shell membrane, 

 the two layers of which are separated at the broad end of the shell to 

 form an air-chamber. This chamber grows larger as development pro- 

 ceeds, and is of some importance in connection with respiration, as an 

 intermediate region between the embryo and the external medium. 

 Beneath the shell membranes lies the albumen, or "white of egg," 

 which is secreted by the thin- walled region of the oviduct ; in it lie two 

 spirally-twisted cords or chalazae, produced by the rotation of the egg in 

 the oviduct. Within the enveloping 

 albumen lies the ovum proper, with 

 its enormous mass of yolk. The 

 yolk is not homogeneous, but con- 

 sists of two substances, known re- 

 spectively as white and yellow yolk. 

 The white yolk forms a central flask- 

 shaped mass, and occurs also as thin 

 concentric layers in the yellow yolk. 



The minimum temperature at 

 which a hen's egg will develop nor- 

 mally is 28 C. If the temperature 

 fall below this, development stops. FlG - 379- Diagrammatic section 



In early stages the interruption may of egg. After Allen Thomson, 

 last for days without fatal results, g.v., Position of germinal vesicle; 

 though always with a tendency to * air-chamber; Y., yolk (al- 

 induce subsequent ^ abnormalities. ^^S*,^^ 

 Towards the end of incubation more 

 than a day's cooling is usually quite fatal. 



On the upper surface of the yolk, in whatever position the egg be 

 held, lies the segmented blastoderm, whose exact origin we must con- 

 sider more precisely. 



As we have seen, yolk is to be regarded as an inert and passive sub- 

 stance. In the hen's egg we have an increased specialisation along 

 the line indicated by the egg of the frog. For there is a small patch 

 of formative protoplasm at one pole, and a large aggregate of yolk 

 composing the remainder of the egg. In consequence, the activity of 



3. Diagrammatic surface view. ./., Area pellucida ; a.o. area 



opaca; ./., neural groove ; fl.s., primitive streak ; M., meso- 

 blast spreading over yolk. 



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



a.0., area opaca; m.s., mesoblast segments; p.s., primitive 

 streak. The dark border shows the spreading of the mesoblast 

 over the yolk. 



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



N., notochord ; w.jj., mesoblastic plates; A. t aorta; Am., 

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



6. Embryo. Cb., Cerebellum ; f. t ear ; //., heart J./C/., fore-limb ; 



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

 #., eye ; C., cerebrum. On the dorsal surface the mesoblastic 

 somites are indicated. 



