EMBRYOLOGY 205 



alimentary canal, and is consequently lined inside by hypoblast. 

 The sac extends forwards into the peritoneal cavity, until it reaches 

 the stalk connecting the embryo with the yolk-sac, whereupon it 

 grows rapidly, and pushes its way into the space between the true 

 and false amniotic sacs. Curving over the embryo, the allantois 

 comes to lie partly above the embryo, separated from the shell by 

 nothing more than the thin false amnion. Being thus situated most 

 superficially, and in close proximity to the air which penetrates the 

 porous shell, the allantois, besides acting as a receptacle for the 

 urine, becomes highly vascular, and i:)erforms the functions of a 

 respiratory organ. Towards the end of incubation, when the 

 embryo is already able to breathe through its lungs, the allantois 

 shrivels up and is cast off', together with the shell, but its narrowed 

 and elongated stalk, from the gut to the navel, remains for some 

 time as the urachus upon the inside of the abdominal wall. 



Chronological and Special Account of the Development of tJte Embryo 



of the Common Fowl. 



First day. \st to 8th hour of incubation. — Scattered cells appear 

 between the epiblast and hypoblast, as the beginning of the middle 

 layer or mesoblast ; they are confined to the posterior part of the 

 area pellucida, and cause this part, called now the embryonic shield, 

 to become somewhat opaque. 



8th to I2th hour.- — The three embryonic fundamental layers 

 are more distinctly established ; the embryonic shield grows fainter, 

 and vanishes after there has appeared within it, through a thicken- 

 ing of the median portion of the blastoderm, the primitive streak, which 

 is a structure of significance still little understood. The hitherto 

 pellucid area becomes oval, its narrow end corresponding Avith the 

 future hind end of the embryo. If an egg be placed with its broad 

 end to the right hand of the observer, the head of the embryo will 

 in nearly all cases be found pointing away from him. 



I2th to 16th hour. — The pellucid area becomes pear-shaped; the 

 primitive streak is marked by a shallow median longitudinal furroAv, 

 known as the primitive groove. 



16^^ to 20th hour. — An important structure, the notochoixl, found 

 in all vertebrate animals, makes its appearance in the median line 

 in front of the primitive streak. The axial part of the epiblast, 

 above the notochord, and in front of the streak, forms two longi- 

 tudinal folds, which enclose the medullary groove. In front of this 

 groove appears the semilunar headfold, and in front of this again 

 the amniotic fold begins to make its appearance. 



20th to 24:th hour. — The semilunar headfold enlarges rapidly, 

 and rises above the level of the blastoderm ; the medullary folds 

 come into contact with each other on the dorsal side, and tend to 



