FOWL— LATER DEVELOPMENT 



555 



Up to about the eleventh day the contraction's of the amnion 

 remain very active, but thereafter they gradually become more 

 gentle until during the closing days of incubation they stop. 

 The mesonephros also attains to its maximum activity and there 

 commences the process of degeneration which will continue till the 

 time of hatching : tubules have developed throughout the length of 

 the metanephros. 



By the twelfth day the duct of the pituitary body has become 

 reduced to a solid cellular strand: the exact time at which this 

 happens is very variable ; it may be as early as the sixth or seventh 

 day. The lachrymal duct, which originated as a solid ingrowth of 

 ectoderm along the line of the lachrymal groove, now becomes tubular. 

 About the twelfth or thirteenth day the cavity reappears over the 

 greater part of the rectum except just at the hinder limit of 

 the occluded portion immediately in front of the allantois. Here 

 the cavity remains blocked till nearly the time of hatching. 



FlQ. 245. — View of head of Fowl embryo as seen from below. (After Duval, 1889.) 



A, five days ; B, six days ; C, eight days, f.n, fronto-nasa'l process ; wu', maxillary process ; olf, olfac- 

 tory opening ; o.n, oro-nasal groove ; sp, hyomandibular cleft ; V, ventricle ; I, II, visceral arches. 



About the thirteenth day the cartilaginous skeleton is complete 

 and the rudiments of claws begin to develop. 



About the fifteenth day the Eustachian valve develops in the 

 heart. 



By the sixteenth day the albumen has all gone and the yolk-sac 

 wall becomes completed ventrally. 



About the nineteenth day the yolk-sac becomes enclosed within 

 the body- wall and the partition between mesenteron and proctodaeum 

 breaks down so that the alimentary canal communicates with the 

 exterior. 



About the twentieth day the umbilicus closes. The violent 

 struggles of the young bird cause its beak to penetrate the air-space : 

 its lungs are filled with air : its further struggles cause its beak to 

 break the shell and it emerges, leaving behind the broken shell lined 

 with the cast-off allantois and serous membrane. 



Correlated with the process of hatching important changes take 

 place in the circulation : the gap in the atrial septum (foramen 



