x FOWL— FIFTH AND SIXTH DAYS 551 



traced dimly back into the region, of the kidney. The main blood- 

 stream to the heart comes from the vitelline vein (v.v) and is joined 

 within the substance of the liver by the blood from the left allantoic 

 vein (a.v) and the posterior vena cava (p.v.c). 



Ignoring the vitelline and allantoic vessels which are clearly 

 adaptations to the peculiar conditions of the developing embryo the 

 main plan of the blood system is seen to be clearly the same as is 

 characteristic of Fishes. 



By cutting off the head after fixing and viewing it from below 

 (Fig. 245, A) the modelling of the face can be studied. The fronto- 

 nasal process (/.«) is bounded on each side by the shallow oro-nasal 

 groove connecting it with the buccal cavity. The ridge forming the 

 outer boundary of the olfactory organ is demarcated from the 

 maxillary process by a faint transverse groove passing outwards 

 towards the eye — the lachrymal groove. Posteriorly the stomodaeal 

 opening is bounded by the mandibular ridge with a distinct break in 

 the middle line between the two mandibular arches. 



Of other developmental features of the fifth day we may note the 

 following. The first indications of turbinals appear on the mesial 

 wall of the olfactory organ, and of semicircular canals in the otocyst. 

 The optic stalk becomes solid : the rudiments of the ocular muscles 

 become recognizable. The pituitary body begins to form outgrowths. 

 The rudiments of thymus and bursa fabricii make their appearance : 

 the bronchi begin to develop branches. The formation of new 

 mesonephric tubule rudiments comes to an end and the mesonephros 

 begins to show signs of functional activity. The atrial septum 

 develops secondary perforations. The fourth aortic arch on the left 

 side, and the portions of aortic root immediately behind the third 

 arch undergo reduction. The horizontal septum of the ventral aorta 

 begins to extend back into the conus and the anterior portions of the 

 posterior cardinal veins begin to undergo atrophy. 



Sixth Day. — During the sixth day of incubation the body of the 

 embryo increases rapidly in size and in correlation with this it dips 

 down into the very fluid yolk, pushing the splanchnopleure of the 

 yolk-sac wall in front of it, so that it is almost hidden from view 

 when the egg is first opened. The amnion is now raised up from the 

 body of the embryo by a marked accumulation of amniotic fluid 

 '(Fig. 242). The allantois has increased greatly in size and in the 

 natural condition is flattened mushroomwise against the inner surface 

 of the serous membrane. In the embryo excised as directed on p. 513 

 it will be seen that the somatopleure of the embryonic body is 

 completely closed in ventrally except for a small circular space round 

 which it is reflected outwards in a funnel-like fashion and continued 

 into the thin membranous amnion. Through the funnel-like opening 

 a slender probe, can be passed from the extra-embryonic coelomic 

 space beneath the serous membrane into the portion of coelome 

 enclosed within the body of the embryo which will become the 

 definitive splanchnopleure or body -cavity. Through the opening 



