AVES. 185 



nearly the whole length of the trunk. This ridge is known as 

 the Wolffian ridge. The first trace of the limbs can be seen 

 towards the end of the third day ; and their appearance at the 

 end of the fourth day is shewn in fig. 1 18, W and HL. 



A section through the trunk of the embryo on the fourth day 

 is represented in fig. 119. The section passes through the region 

 of the trunk behind the vitelline duct. The mesentery (M) is 

 very much deeper and thinner than on the previous day. The 

 notochord has become invested by a condensed mesoblastic 

 tissue, which will give rise to the vertebral column. The two 

 dorsal aortae have now completely coalesced into the single 

 dorsal aorta, and the Wolffian body has reached a far more 

 complete development. 



In the course of the fifth day the face begins to assume a less 

 embryonic character, and by the sixth and succeeding days 

 presents distinctive avian characters. 



The general changes which take place between the sixth 

 day and the time of hatching do not require to be specified in 

 detail. 



Fcetal Membranes. 



The Reptilia, Aves and Mammalia are distinguished from 

 the Ichthyopsida by the possession of certain provisional fcetal 

 membranes, known as the amnion and allantois. 



As the mode of development of these membranes may be 

 most conveniently studied in the Chick, I have selected this 

 type for their detailed description. 



The Amnion. The amnion is a peculiar sack which en- 

 velopes and protects the embryo. 



At the end of the first day of incubation, when the cleavage of 

 the mesoblast has somewhat advanced, there appears, a little way 

 in front of the semilunar head-fold, a second fold (fig. 102, also fig. 

 121 C, d/ and fig. 122, Am], running more or less parallel or rather 

 concentric with the first and not unlike it in general appearance, 

 though differing widely from it in nature. This second fold 

 gives rise to the amnion, and is limited entirely to the somato- 

 pleure. Rising up as a semilunar fold with its concavity directed 

 towards the embryo (fig. 121 C, of], as it increases in height it 



