AVES. 



45 



and serves to extract the food for the use of the embryo and 

 to aerate the blood before the lungs become of use. The atn- 

 nion and allantois (see 415 ; Fig. 205) are both developed as 

 in reptiles. Of these the amnion appears first. By a study of 

 Figs. 204 and 205, together with others in the reference texts, 

 it will be seen that the amnion is an outgrowth of the body- 

 wall of the embryo and has a cavity continuous with the 

 coelom. The outer layer is known as the false amnion; the 

 inner is the true amnion (Fig. 204, am 2 , am 1 ). Into the space 

 between the amnion-layers the wall of the gut outpockets, 



FIG. 205. 

 am.c an? 



am 



CO. 



al. 



FIG. 205. Diagram of a longitudinal section through the embryo of a fowl, show- 

 ing formation of amnion and allantois and the relation of these membranes to the 

 embryo. The boundaries are as in the preceding figure, am 1 , inner or true amnion; 

 am 2 , outer or false amnion; am.c., amniotic cavity; al., allantois; c.c., central canal of 

 the spinal cord; co., ccelom; g, gut; ys., yolk sac. 



Questions on the figure. Which of the three embryonic layers enter 

 into the amniotic folds? Which go to form the allantois? Show that 

 the cavity between the true and false amnion is " extra-embryonic " coelom. 

 How is the amniotic cavity lined? With what is the yolk sac lined? The 

 cavity of the allantois is in reality a portion of what cavity? Which of 

 these membranes unite in mammals to form the chorion (see 451)- 



