ANATOMY. 



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distinct. The mouth is formed by perforation of the oral 

 membrane, a thin partition between the pharynx and a 

 depression from the surface known as the stomodseum. 



B. Anatomy. 



The most important feature is the commencing develop- 

 ment of all the internal organs. The third day is the most 

 eventful date of the whole incubation period. 



aa. Alimentary Canal. 



The pharynx undergoes a transformation ; four pairs of 

 pouches grow outwards towards the surface of the head. 



Fig. 90. A transverse section across the body of a Chick Embryo at 

 the forty-eighth hour of incubation, x 150. 



A, aorta. AN, amnion, c, body cavity or CKlom. CH, notochord. CM, 

 myocoel, or cavity of mesoblastic somite. H, hypoblast forming roof of mid- 

 gut. KC, Wolffian duct. KS, nephrostome. ME, somatopleuric layer of 

 mesoblast. MH, splanchnopleuric layer of mesoblast. MP, muscle plate. 

 NE, rudiment of spinal ganglion. NS, spinal cord. OE, genital epithelium. 

 VC, posterior cardinal vein. VV, vitelline vein. 



These outgrowths meet with corresponding ingrowths from 

 the surface. The first pouch is the hyomandibular, and gives 

 rise to part of the Eustachian tube and tympanic cavity. 



