192 



THE FIFTH DAY. 



[CHAP. 



The external constriction actually dividing the bulbus 

 into two vessels, does not begin to appear till the septum has 

 extended some way back towards the heart. 



The semilunar valves become pocketed at a period con- 

 siderably later than their first formation (from the 147th to 

 the 165th hour) in the order of their appearance. 



8. Towards the end of the fifth and in the course of the 

 sixth day further important changes take place in the heart. 



The venous end with its two very conspicuous auricular 

 appendages, comes to be situated more above (dorsal to) the 

 arterial end, though it still turns rather towards the left. 

 The venous portion of the heart undergoes on the sixth day 

 or even near to the end of the fifth, such a development of the 

 muscular fibres of its walls, that the canalis auricularis becomes 

 almost entirely concealed. The point of the heart is now 



ri> 



Fig. 6o. 

 A 



TWO VIEWS OF THE HEAET OF A CHICK UPON THE FlFTH DAT OF INCUBATION, 

 A from the ventral, B from the dorsal side. 



I. a. left auricular appendage, r. a. right auricular appendage, r.v. right 

 ventricle. I. v. left ventricle, b. bulbus arteriosus. 



