196 FCETAL MEMBRANES. 



two forward streams reaching a gap in the front part of the 

 sinus terminalis fall into either one, or in some cases two veins, 

 which run straight backwards parallel to the axis of the embryo, 

 and so reach the roots of the heart. When one such vein only 

 is present it joins the left vitelline trunk ; where there are two 

 they join the left and right vitelline trunks respectively. The 

 left vein is always considerably larger than the right ; and the 

 latter when present rapidly gets smaller and speedily disap- 

 pears. After the third day, although the vascular area goes 

 on increasing in size until it finally all but encompasses the 

 yolk, the prominence of the sinus terminalis becomes less and 

 less. 



The fcetal membranes and the yolk-sack may conveniently 

 be treated of together in the description of their later changes 

 and final fate. 



On the sixth and seventh days they exhibit changes of great 

 importance. 



The amnion, at its complete closure on the fourth day, very 

 closely invested the body of the chick : the true cavity of the 

 amnion was then therefore very small. On the fifth day fluid 

 begins to collect in the cavity, and raises the membrane of the 

 amnion to some distance from the embryo. The cavity becomes 

 still larger by the sixth day, and on the seventh day is of very 

 considerable dimensions, the fluid increasing with it. On the 

 sixth day Von Baer observed movements of the embryo, chiefly 

 of the limbs ; he attributes them to the stimulation of the cold 

 air on opening the egg. By the seventh day very obvious 

 movements begin to appear in the amnion itself; slow ver- 

 micular contractions creeping rhythmically over it. The amnion 

 in fact begins to pulsate slowly and rhythmically, and by its 

 pulsation the embryo is rocked to and fro in the egg. This 

 pulsation is probably due 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. 

 Similar movements are also seen in the allantois at a consider- 

 ably 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 



