DEVELOPMENT 



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II. Hypoblast. — Epithelia of (a) the alimentary canal from 

 the back of the mouth to the anus and of all its glands ; (6) of 

 the Eustachian tube and tympanum ; (c) of the trachea and 

 lungs ; {d) of the thyreoid and thymus ; and (e) of the urinary 

 bladder and urethra. 



III. Mesoblast. — All other structures. 



By the formation of a vertical groove down the back of the 

 blastoderm, a tube of epiblast cells (the neural canal) is enclosed, 

 from which the nervous system develops by the conversion of 



Fig. 241. — Longitudinal Section through the Human Uterus and Ovum at the 

 Fifth Week of Pregnancy. D.S., decidua serotina, which will become 

 the placenta ; D.R., decidua reflexa ; D. V., the uterine mucous mem- 

 brane called the decidua vera. 



some of the cells into neurons, and others into neuroglia cells 

 (fig. -239). 



The mesoblast on each side of this splits, and the outer part, 

 with the epiblast, goes to form the body- wall (Somatopleur), while 

 the inner part with the hypoblast gets tucked in to produce 

 the alimentary canal (Splanchnopleur) (fig. 239). 



The developing embryo sinks into the blastocyst, and, as a 

 result of this, the somatopleur folds over it and, uniting above, en- 

 closes it in a sac — the amniotic sac (fig. 240, am.), which becomes 

 distended with fluid — the amniotic fluid, in which the embryo 

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