TISSUE FORMATION. 65 



those simple forms of life already described, which in- 

 crease by fission or budding, (Chap. III., Sec. 12, 13,) the 

 germ of all the tissues is first a piece of simple bioplasm 

 derived from the vesicles of the ovary. This is fertilized 

 by fusion with similar bioplasm derived from the male. 

 It then acquires a membrane, and exhibits a nucleus 

 and nucleolus, as in the case of the primitive vegetable 

 cell. Changes, however, take place in the animal ovum 

 which we do not observe in the vegetable, and these 

 changes differ also in the different classes of animals. 

 In the higher classes the ovum separates into two 

 spheres, which sub-divide into four, then into a mul- 

 berry-like mass of cells, or morula. (Fig. 20.) These cells 



FIG. 20. Segmentation of Mammalian Egg. A. Division into halves. B. Further 

 subdivision. C. Mulberry mass, or Morula. 



in the vertebrates arrange themselves into a layer lining 

 the vitelline membrane, on one side of which is a sort of 

 pouch, or blastoderm, consisting of three layers of cells, 

 the epiblast, the mesoblast, and the hypoblast. The 

 first of these produces the skin, the middle one the 

 nervous, muscular, and vascular systems, and the latter 

 the lining of the intestinal and respiratory organs. 



The alimentary canal is at first a straight tube closed 



at both ends. As it grows faster than the body it is 

 6* 



