1'.' 



pumping the blood of the omphalo'iMSwaio veins, it follows that both expan- 

 sion and contraction is necessary for accomplishing this, the one for aspirating 

 this fluid, tin- other for pn>|>ellini; it. This would explain why the action should 

 In-ill a <il this r/(//(i////c hi urt. ( )f course, as the area of circiilntioii increases it 

 would c.-Jl for corresjKMidmg increase of force for effecting it. Accordingly, prea- 

 "I for facilitating circulation at the same time that additional force 

 i> placed ii]x)ii it ; notablv, the former is produced \>\ tin 1 ainniotic fluid and the 

 latter by the action in the ainnion. 



Tin' aiuiiion closes around the embryo of the chick on the fourth day, 

 and on the fifth tluid U^'ins to collect in the sac, and by the seventh the 

 embryo is submerged in a considerable quantity of water. "By the seventh 

 day very obvious movements U'gin to api>ear in the amnion itself : slow vermic- 

 ular contractions creep rhythmically over it. The amnion, in fact, begins to 

 pulsate slowly and rhythmically, and by its pulsations 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 

 jHirtion of the mesoblast, forming part of the ainniotic fold" (Foster and 



K-llfoUM. 



The physiological significance of this accumulation of amniotic fluid, and the 

 rhythmical contractions and expansions in the amnion, may not be doubted for 

 a single moment, since the former would increase pressure, while the latter 

 should produce the necessary changes in pressure in the embryo for compelling 

 respiration and circulation to be in correspondence with the nutritive and func- 

 tionid processes in the growing chick, both of which are constantly extending 

 their limits and requiring more and more force for effecting them. These 

 slow pulsations in the amuion of the chick answer to the placental and 

 uterine souffle in gestation, the principle being precisely the same. How other- 

 wise explain this circumstance ? But, as has already been remarked, a II pulsa- 

 tion* relate to changes in pressure, and these pulsations in the amnion, together 

 with the amniotic fluid, relate to changes in pressure in the embryo for increas- 

 ing circulation of the juices. 



The explanation of the mechanics is sufficiently easy ; notably, there are two 

 cardinal points from which to regard it one in the allantois, the other in the 

 embryo. First, commencing with the movement of expansion in the allantois. 

 The increase in pressure which this produces in the embryo through the am- 

 niotic fluid should cause the venous blood to flow with increased energy towards 

 the allantois, the point of low pressure within the egg (the heart and vascular 

 >\ stem, of course, assisting in this); the contraction or condensation of the 

 amnion. by relieving pressure in the allantois, enables this to expand part passu 

 with contraction in the amnion for aspirating the venous blood, at the same 

 time that it aspirates the air through the outer membrane and pores of the shell. 

 When the movement is reversed by expansion of the amnion, the reduction in 

 pressure which this effects in the embryo, together with the simultaneous in- 

 crease of pressure it produces in the allantois by forcibly compressing this 

 against the shell wall, causes the aerated blood in the latter to flow with aug- 

 mented speed into the heart of the embryo, as also through the tissues of the 



