THE EARLY DEVELOPMENT OF THE CHICK 295 



originally the median plane. That is, they continue to close 

 together along what was originally marked out as the mid-line, 

 and they finally close immediately above the right hind-limb 

 bud. It follows from this that the left lateral fold becomes 

 much the more extensive, passing from the lower (left) side of 

 the embryo, around to the upper (right) side and thus covering 

 the whole morphologically dorsal surface of the embryo, while 

 the right lateral fold covers only a portion of the upper (morpho- 

 logically right) surface of the embryo. This is correlated with 

 the fact that the right fold is thicker than the left and is usually 

 thrown into irregular transitory folds or wrinkles. 



Soon after the amnion is fully formed, muscle fibers are 

 differentiated in its mesodermal layer. Waves of contraction 

 then slowly pass along the amnion and keep up a movement of 

 the amnionic and coelomic fluids. It is also supposed that 

 they keep the embryo in slight motion, preventing adhesions 

 between the embryo and its appendages. The amnion also 

 serves, probably, as a water cushion, protecting the embryo 

 against deforming pressures. The later relations of the 

 amnion to the allantois and the albumen of the egg will be 

 mentioned below. 



3. The Allantois 



The allantois is an embryonic appendage of the greatest 

 importance. Unlike the yolk-sac, amnion, and chorion, it is 

 developed from the embryo itself, and not from the extra- 

 embryonic structures. When fully developed it is the most 

 extensive of all these accessory organs, the arrangement of 

 which becomes considerably modified by its growth. Function- 

 ally, too, it is of the utmost importance, it is primarily the 

 embryonic respiratory organ, but it serves also as a reservoir 

 for excretory products, and through the development of muscle 

 fibers in its wall it effects certain movements of the embryo, 

 and assists in the final inclusion of the yolk-sac within the 

 embryonic body. 



The morphological relations of the allantois to the embryo 



