REPRODUCTION 



435 



The latter spread out at one pole to form the blastoderm (fig. 

 170, A) and dispose themselves in three layers the epiblas't, 



FIG. 169. Ovum after segmentation, showing the formation of the Ectoderm 

 (a) and Endoderm (6). From the cells of the latter the Blastoderm is 

 formed. (ELLENBERGER. ) 



mesoblast, and hypoblast (fig. 170, B and C). From these 

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



I. Epiblast. Nervous system, epidermis and appendages. 

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

 and glands opening into them, and the enamel of teeth. 



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

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



A , B c 



FIG. 170. To show A, the spreading out of the Endoderm cells to form the 

 Blastoderm ; B, the formation of Epiblast and Hypoblast ; and C, of 

 Mesoblast. In B and C the ectoderm is not seen. (ELLENBERGER.) 



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

 (d) of the thyroid 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 



