4i6 



HUMAN PHYSIOLOGY 



blastoderm, and dispose themselves in three layers the 

 epiblast, mesoblast, and hypoblast (Fig. 158). From these 

 layers the various parts of the body are derived as follows : 

 I. Epiblast. Nervous system, epidermis, and appendages. 

 Epithelium of mouth, nose, naso-pharynx, and all cavities 



and glands opening into them, 

 and the enamel of teeth. 



II. Hypoblast. Epithelia 

 of (a) alimentary canal from 

 back of mouth to anus and of 

 all its glands ; (b) of Eustachian 

 tube and tympanum ; (c) of 

 trachea and lungs ; (d) of 

 thyroid and thy m us ; and 

 (e) of 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 



FIG. 158. Transverse section of more 

 advanced Blastoderm, to show Epi- 

 blast, Mesoblast, and Hypoblast, 

 formation of Neural Groove and 

 splitting of the Mesoblast. 



FlQ. 159. Longitudinal Section through Embryo to it sinking down into ovum and 

 the formation of the Amnion, am. In the Mesoblast round, all., the Allantois, 

 the blood vessels grow out to form the placenta. 



conversion of some of the cells into neurons, and others 

 into neuroglia cells (Fig. 158). 



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 pro- 

 duce the alimentary canal (Splanchnopleur) (Fig. 158). 



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

 result of this, the somatopleur folds over it and, uniting 

 above, encloses it in a sac the amniotic sac (Fig. 159, am.), 

 which becomes distended with fluid the amniotic fluid, in 

 which the embryo floats during the later stages of its develop- 



