Development of the Third Day 169 



so abundant that the embryo can move freely 

 in the amniotic cavity ; and a rocldng motion 

 is given the embryo by the contraction of 

 muscle fibres that are developed in the am- 

 niotic membrane. What the purpose of this 

 rocking motion may be is not easy to say, but 

 the chief object of the amnion and its con- 

 tained fluid is probably for the protection of 

 the soft and delicate embryo. 



The amnion is not really a part of the embryo 

 proper, and is left in the shell at the time of 

 hatching. It is a very characteristic structure 

 in the development of the three higher groups 

 of Vertebrates, — -Reptiles, Birds, and Mammals. 



The allantois. — Although the allantois origi- 

 nates during the second day and continues to 

 increase in size throughout a greater part of 

 the period of incubation, it will be convenient 

 to describe its complete history at this point, 

 so that it need be merely mentioned at sub- 

 sequent periods. We cannot do better than 

 quote at length from the account of the de- 

 velopment of the allantois given by Marshall. 



" The allantois is really an appendage of the alimen- 

 tary canal, arising as an outgrowth of its ventral wall, in 

 front of the cloaca; it is therefore lined by hypoblast, 

 like all other outgrowths of the mesenteron, while the 



