CHAP. VIII.] THE rCETAL APPENDAGES. 201 



to develope into a beak ; and the commencing bones of the 

 skull arrange themselves after an avian tjrpe. Into these 

 details we do not propose to enter, and shall therefore 

 treat the history of the remaining days with great brevity. 



We will first speak of the fcetal appendages. 



2. On the sixth, and seventh days, these exhibit changes 

 which are hardly less important than the events of previous 

 days. 



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 vermicular contractions 

 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. (Cf. 

 Chap. II. § 9, p. 42.) 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 chorion). 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 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. 



