DEVELOPMENT OF THE CHICK. 



659 



the oviduct. Within the enveloping albumen lies the ovum proper, with 

 its enormous mass of yolk. The yolk is not homogeneous, but consists 

 of two substances, known respectively 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 normally 

 is 28 C. If the temperature fall below this, development stops. In 

 early stages the interruption may last for days without fatal results, 

 though always with a tendency to induce subsequent abnormalities. 

 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 consider 

 more precisely. 



As we have seen, yolk is to be 

 regarded as an inert and passive 

 substance. 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 com- 

 posing the remainder of the egg. 

 In consequence, the activity of the 

 protoplasm is unable to overcome 

 the inertia of the yolk, and segmenta- 

 tion is meroblastic and discoidal (cf. 

 Elasmobranchs). 



In the protoplasm of the egg hori- 

 zontal and vertical furrows appear in rapid succession. The result, as 

 exhibited by vertical sections, is to produce an upper epithelial layer 

 of cells, separated by a small space from larger, more irregular cells, 

 which are still in connection with the yolk on which they lie. At the 

 circular border of the germinal disc the two sets of cells are continuous. 

 According to some authorities, this stage represents the blastula, the 

 upper layer of cells corresponding to the cells of the animal pole in the 

 frog, the lower with the enormous mass of yolk on which they lie to 

 the cells of the vegetative pole, the space to the segmentation cavity. 



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



opaca ; ./., neural groove ; p.s., primitive streak; J/. , meso- 

 blast spreading over yolk. 



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



a.o., area opaca; in.s., mesoblast segments; p.s., primitive 

 streak. The dark border shows the spreading of the mesoblast 

 over the yolk. 



5. C'ross-section. s.c. , Spinal cord ; s.g., rudiment of spinal ganglia ; 



A T ., notochord ; in. p., mesoblastic plates; A., aorta; Am., 

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



6. Embryo. C/'., Cerebellum ; F., ear ; //., heart ',f.l., fore-limb ; 



/i. 1., hind-limb ; y.s., stalk of cut-off yolk-sac ; A I., allantois ; 

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

 somites are indicated. 



FIG. 329. Diagrammatic section 

 of egg. After Allen Thomson. 



g.v., Position of germinal vesicle ; 

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

 nate layers of "yellow ' and 

 "white"); ch., chalaza. 



