226 



Till-: chick 







At thirty-six hours, the folding off of the embryo from the 

 yolk-sac has only made slight progress ; the head of the embryo 

 (Fig. 112) is lifted up above the yolk-sac by an anterior con- 

 striction or head fold, but the sides and tail end are as yet only 

 very imperfectly defined. 



By the end of the third day great advance has been made. 

 The embryo (Fig. 99) has increased considerably in size. In 

 the head, which has grown much faster than the body, and is 

 now disproportionately large (Fig. 113), the nose, eye, and 

 ear, and the several divisions of the brain, are well established. 

 The head is no longer straight, but is strongly flexed, owing to 

 the dorsal surface growing much more rapidly than the ventral. 

 The heart and blood-vessels have acquired definite and charac- 

 teristic arrangement. The folding off of the embryo from the 

 yolk-sac has made considerable progress (Fig. 114); the head 

 and neck are now quite free from the yolk-sac - 2 the hinder end 

 of the embryo is lifted up from the yolk by a definite tail fold 

 (Fig. 114, tl), and the side walls of the embryo are much more 

 clearly defined. The yolk-stalk, connecting the embryo with 

 the yolk-sac, is now a short tube, the diameter of which is about 

 a third of the length of the embryo. The hinder end of the 

 embryo still lies with its dorsal surface facing the egg-shell, and 

 its ventral surface resting on the yolk-sac ; but the head and neck 

 have rolled over, so as to lie with their left side on the yolk-sac 

 and their right side towards the egg-shell ; the axis of the body 

 becoming spirally twisted in consequence (Fig. 113). 



(On the fouxtk_day:) the folding off of the embryo makes 

 further progress, and the yolk-stalk becomes greatly narrowed. 

 The whole embryo becomes strongly flexed, the dorsal surface 

 being convex along its entire length. The body, as well as the 

 head, of the embryo now lies with its left side on the yolk-sac ; 

 and the rudiments of the limbs have appeared as two pairs of 

 ^small, ill-defined buds from the sides of the body. 



By the end of the fifth day the embryo has acquired the 

 shape and proportions shown in Fig. 100. In the natural condi- 

 tion, it lies with its left side on the yolk-sac, with which it is 

 connected by the narrow tubular yolk-stalk. The whole embryo 

 is strongly flexed, the convex dorsal surface being about four 

 times the length of the concave ventral surface. .The head is of 

 relatively enormous size, chiefly owing to the great development 



