The Adult Tissues of the Human Body 23 



. i 

 The egg cell is transformed by repeated mitosis into the morula, a 



spheroidal mass of cells uniform in appearance [Fig. 9, C]. From this 

 compact mass is next developed the blastula, a hollow vesicle, the cells 

 being distributed to form at first a wall of a single layer, and then by con- 

 tinued multiplication forming three layers; the ectoderm or outer layer, 

 the endoderm or inner layer, and the mesoderm or middle layer. 



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Fig. io. Stratified epithelium from esophagus of cat. Highly magnified. (Bailey, 

 Histology. ) 



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Fig. II. Stratified ciliated epithelium from human trachea. Highly magnified. 

 (Bailey, Histology.) The ciliated cells are columnar. One 'goblet cell' (cell secret- 

 ing mucus) is shown. 



The ectoderm and the endoderm are epithelia, composed of cells of 

 compact form more or less closely arranged. The mesoderm is in part 

 made up of cells like those of the two other layers, and in part of branch- 



