FOWL— SIXTH TO EIGHTH DAYS 



553 



pro- 



The main portions of the skeleton become laid down 

 chondral tissue and, towards the end of the day, in cartilage. 



The heart begins to assume its definitive external form; the 

 ventricular septum develops and the conus septum begins to do so. 

 The fourth aortic arch becomes obliterated on the left side. 



Seventh Day (Figs. 243 and 244).— The mushroom - shaped 

 allantois is spreading actively all round beneath the serous 

 membrane. The amnion is beginning to show waves of contraction 

 passing along its wall. The brain and eyes and consequently the 

 head as a whole are of relatively enormous size. In sections the roof 

 of the fourth ventricle is 

 found to be developing ir- 

 regular folds in which the 

 vessels of the choroid plexus 

 will appear. All three 

 turbinal rudiments are pre- 

 sent in the nose. The crop 

 is beginning to expand. The 

 visceral clefts are all closed. 

 The glands of the stomach 

 are beginning to make their 

 appearance as rudiments. 

 The cavity of the enteron 

 disappears for some distance 

 forwards from the point of 

 origin of the allantois. The 

 Miillerian ducts may show 

 incipient asymmetry. The 

 notochord is beginning to 

 be constricted by the verte- 

 brae. The first traces of 

 ossification are making their 

 appearance, especially in the 

 skeleton of the limbs. 



The septum of the conus 

 arteriosus is complete and the muscular coat extends into it from 

 each side : the pocket-valves are becoming excavated. The fourth 

 aortic arch on the left side has disappeared while the portion of 

 aortic root between arches III and IV on the right side, and behind 

 arch III on the left side, are becoming obliterated. 



Eighth Day. — The movements of the amnion now reach their 

 highest degree of activity. The fronto-nasal process (Fig. 245, C) is 

 growing out to form the pointed beak while the lower jaw is taking 

 a similar pointed form, the two mandibular arches being now con- 

 tinued into one another ventrally without a break. The rudiments 

 of feathers are beginning to make themselves apparent. 



In the brain the cerebellum is becoming folded on itself so as to 

 bulge outwards. The oro-nasal grooves are covered in to form the 



Fig. 243. — Fowl's egg opened during the seventh day. 

 The tody of the chick is seen dimly through the 

 highly vascular allantois. The vessels of ,the 

 allantois can be distinguished from those of the 

 vascular area by their turning back at the edge of 

 the allantois while those of the vascular area pass 

 onwards uninterruptedly. The highly fluid char- 

 acter of the yolk is shown by the yolk-sac wall 

 bulging outwards over the broken shell at the 

 point marked *. 



all, allantois. 



