276 THE SIXTH DAY. [CHAP. 



We do not mean that on the sixth day all the organs 

 suddenly commence to exhibit peculiarities which mark 

 them as avian. There are no strongly marked breaks 

 in the history of development; its course is perfectly 

 gradual, and one stage passes continuously into the 

 next. The sixth and seventh days do however mark 

 the commencement of the period in which the spe- 

 cialization of the bird begins to be apparent. Then for 

 the first time there become visible the main features 

 of the characteristic manus and pes ; the crop and the 

 intestinal caeca make their appearance ; the stomach 

 takes on the form of a gizzard ; the nose begins to de- 

 velope into a beak ; and the commencing bones of the 

 skull arrange themselves after an avian type. Into 

 these details we do not propose to enter, and shall 

 therefore treat the history of the remaining days with 

 great brevity. 



We will first speak of the foetal appendages. 

 On the sixth and seventh days these exhibit 

 changes which are hardly less important than the 

 events of previous days. 



The amnion at its complete closure on the fourth 

 day very closely invested the body of the chick; the 

 true cavity of the amnion was at that time therefore very 

 small. On the fifth day fluid begins to collect in the 

 cavity, and raises the membrane of the amnion to some 

 distance from the embryo. The cavity becomes still 

 larger by the sixth day, and on the seventh day is of 

 very considerable dimensions, the fluid increasing wdth 

 it. On the sixth day Von Baer observed movements of 

 the embryo, chiefly of the limbs; he attributes them 

 to the stimulation of the cold air on opening the egg. 



