FOWL— FOUETH AND FIFTH DAYS 



549 



ruesonephros secondary tubules make their appearance while in the 

 anterior region a process of degeneration becomes apparent. During 

 the second half of the day the ureter begins to sprout out from the 

 Wolffian duct and about the end of the day the rudiments of Miil- 

 lerian ducts and of the metanephric units may become recognizable. 



In the heart the atrial septum becomes completed about the end 

 of the fourth day and the endothelial cushions begin to develop. 



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



all, allantois ; C.H, cerebral hemisphere ; E, eye ; By, operculum ; M, mandibular arch ; pin, 

 pineal rudiment faintly visible as slight elevation on roof of thalamencephalon ; Rh, thin roof of 

 rhombencephalon ; som, edge of somatopleure cut through where it becomes reflected back over the 

 body of the embryo to form the amnion ; t.o, roof of mesencephalon (optic lobe) ; V, ventricle ; v.c, 

 visceral clefts III and IV ; y.s, 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 view 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- 



