AVES 



409 



tion. For the details of further development the student must 

 be referred to more extensive texts, but it may be stated that 

 the blastoderm comes to consist of two layers of cells which 

 have been likened to two watch glasses so placed as to enclose 

 a shallow cavity. The outer layer is ectodermal and is continu- 

 ous at the edge with the inner, which is composed of larger cells 

 incompletely separated from the yolk beneath (Fig. 13, C, 4). 

 This inner layer gives rise to both entoderm and mesoderm. The 

 blastoderm continues to grow at the margins until the yolk is 

 entirely enveloped by a living membrane which is well supplied 

 with blood vessels and serves to extract the food for the use of the 

 embryo and to aerate the blood before the lungs become of use. 



FIG. 209. 



FIG. 209. ArchcEopteryx lithographica, an early reptilian Bird. From Claus. 



Questions on the figure. What in the figure shows this to be a bird? Wha* 

 shows it to be different from typical birds? What is signified by each of the term- 

 in its scientific name? 



The amnion and allantois (see 423; Fig. 208) are both de- 

 veloped as in reptiles. Of these the amnion appears first. By 

 a study of Figs. 207 and 208, 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 ccelom. 

 The outer layer is known as the false amnion; the inner is the 

 true amnion (Fig. 207, am 2 , am 1 ). Into the space between the 

 amnion-layers the wall of the gut outpockets, forming the al- 

 lantois (Fig. 208, a/.). The cavity of this sac is continuous 

 with the lumen of the gut. The embryo thus becomes com- 



