AVES 



409 



a layer of albumen, and later is stirrounded by a membranous 

 covering and by a porous, limy shell, all of which are secreted 

 by the walls of the oviduct. The protoplasm is confined to a 

 small germinal disc and segmentation is discoidal, resulting in 

 a blastoderm like that of reptiles. In the newly laid egg cleavage 



Fig. 208. 

 am.c anf 



Fig. 208. Diagram of a longitudinal section through the embryo of a fowl, showing formation 

 of amnion and allantois and the relation of these membranes to the embryo. The boundaries are 

 as in the preceding figure, om^, inner or true amnion; ani^^ outer or false amnion; am.c*t amniotic 

 cavity; a^, allantois; cc, central canal of the spinal cord; co., coelom; g, gut; :V5., 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 §462). , 



is well advanced. After the egg is laid, cleavage is checked 

 until the necessary temperature for further development is 

 supplied either by the brooding of the parent or by some special 

 device. Owing to the action of gravity on the heavier yolk 

 the living disc is always directed upward, — the position most 

 favorable for getting the warmth of the parent's body in incuba- 



