AVES. 163 



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 embr} r o. 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 stimula- 

 tion of the cold air on opening the egg. JBy 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 pro- 

 bably 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 considerably 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, between the amnion proper and the false amnion or 

 serous envelope. It is filled with fluid, so that in spite of its flattened 

 form its opposite walls are distinctly separated from each other. 



The vascular area has become still further extended than on the 

 fifth 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, assuming 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 Von Baer) increased. This can only be due 

 to its absorbing the white of the egg, which indeed is diminishing 

 rapidly. 



During the eighth, ninth, and tenth days, the amnion does not 

 undergo any very important changes. Its cavity is still filled with 

 fluid, and on the eighth day its pulsations are at their height, hence- 

 forward diminishing in intensity. 



The splitting of the mesoblast has now extended to the outer limit 

 of the vascular area, i.e. over about three-quarters of the yolk-sack. 

 The somatopleure at this point is continuous (as can be easily 

 seen by reference to fig. 121) with the original outer fold of the 

 amnion. It thus comes about that the further splitting of the meso- 

 blast merely enlarges the cavity in which the allautois lies. The 

 growth of this organ keeps pace with that of the cavity in which 

 it is placed. Spread out over the greater part of the yolk-sack as a 



112 



