412 EMBRYOLOGY OF THE CHICK. 



niinal layers are present in all animals. From the epiUast 

 arises the epidermis, the central nervous system, and sense- 

 organs ; the mesoblast gives rise to the vascular, muscular 

 skeletal, and connective tissues, excretory organs, and the 

 generative glands. The lowest layer forms the lining of the 

 alimentary canal (except near the mouth and anus, which 

 are lined by epiblast) and the glands attached to the alimentary 

 canal. 



The blastoderm gradually grows round the yolk, and forms a. 

 sac round it by the seventh day of incubation. The area opaca 

 is the part that covers the yolk, not the area pellucida. Whilst 

 this extension of the blastoderm is going on, the embryo chick 

 becomes formed by a folding-off of the central part of the area 

 pellucida. At first there appears a semilunar groove, which, as 

 it were, tucks in a small part of the blastoderm in the form of a 

 crescent — the head-fold. Some time after the formation of the 

 head-fold there appears a similar, but smaller, fold at the opposite 

 end of the disc — the tail-fold — which travels forwards whilst 

 the head-fold passes backwards. Between these two folds two 

 lateral folds appear ; these sink inwards, whilst the head-fold 

 passes backward and the tail-fold forwards, tending to join in 

 the middle line, and thus give rise to a small sac above, con- 

 nected by a continually narrowing neck with a larger sac below. 

 The upper is the "embryonic sac,'' the lower the yolk-sac; 

 as the former grows the latter disappears. The yolk-sac is 

 gradually absorbed for nourishment by the embryo chick being 

 formed in the sac above. Within one or two days of hatching 

 we shall see this yolk-sac disappear into the chick. The em- 

 bryo thus folded off from the yolk-sac gradually thickens, and 

 throws out various processes, swellings, &c., to form the legs, 

 wings, and other parts by unequal growth. Internally more 

 complicated changes go on. On the upper surface is seen a 

 streak, the primitive streak, in which appears a depression, 

 the primitive groove. In front of the primitive groove there 

 is formed a thickening of the epiblast, the medullary plate, the 



