IX.] THE YOLK. 277 



By the seventh day very obvious movements begin to 

 appear in the amnion itself; slow vermicular con- 

 tractions creep rythmically over it. The amnion in 

 fact begins to pulsate slowly and rythmically, and by 

 its pulsation the embryo is rocked to and fro in the 

 egg. This pulsation is due probably to the contraction 

 of involuntary muscular fibres, which seem to be present 

 in the attenuated portion of the mesoblast, forming 

 part of the amniotic fold. (Of. Chap. II. p. 45.) Similar 

 movements are also seen in the allantois at a con- 

 siderably later period. 



The growth of the allantois has been very rapid, 

 and it forms a flattened bag, covering the right side of 

 the embryo and rapidly spreading out in all directions, 

 between the primitive folds of the amnion, that is be- 

 tween the amnion proper and the false amnion (serous 

 membrane). It is filled with fluid, so that in spite of 

 its flattened form its opposite walls are distinctly sepa- 

 rated from each other. 



The vascular area has become still further extended 

 than on the previous day, but with a corresponding loss 

 in the definite character of its blood-vessels. The sinus 

 terminalis has indeed by the end of the seventh day 

 lost all its previous distinctness, and the vessels which 

 brought back the blood from it to the heart are no 

 longer to be seen. 



Both the vitelline arteries and veins now pass to 

 and from the body of the chick as single trunks, as- 

 suming more and more the appearance of being merely 

 branches of the mesenteric vessels. 



The yolk is still more fluid than on the previous 

 day, and its bulk has (according to Yon Baer) increased. 



