FOWL FOURTH AM) FIFTH DAYS 



549 



mesonejthms secondary tubules make their appearance while in the 

 anterior region a process of degeneration becomes apparent. Imriiiir 

 the second halt' of the day the ureter lupins in sprmit out I'mm the 

 WolIHan duct and about the end of the day the rudiments <>t Mul- 

 lerian ducts and of the inetanephric units may become recognizable. 



In the heart the atrial septum becomes completed about the end 

 of the fourth day and the endothelial cushions lie-in t<> develop. 



RA. 



som. 



FIG. 240. Chick extracted from the egg at about the middle of the fifth day of incubation. 



all, allantois ; C.H, cerebral hemisphere ; K, eye ; Hy, operrulum ; .V, mandibular arch ; pin, 

 pineal rudiment faintly visible us slight elevation on roof of thalamencephalon ; Bh, thin roof of 

 rhombencephaloii ; .<m, edge of somatopleure cut through where it becomes reflerti-il hack ovtr tin' 

 body of the embryo to form the amnion ; t., roof of mewnoephalon (oj)tic lobe); I". . 

 visceral clefts III and IV : //.-. yolk-sac. 



FIFTH DAY. The progress in development during the course of 

 the fifth day is illustrated by Figs. 239-241. The albumen has so 

 shrunk in volume as to be no longer visible in a \ ie\\ of the opened 

 egg from above : the yolk has become extremely fluid : the vascular 

 area has increased considerably in size. The allantois is now a con- 

 spicuous object and the mesoderm covering its surface is beginning 

 to develop blood-vessels. The head of the embryo is, as before, of 

 relatively very large size: the flexure in the region of the mesen- 



