66 THE SCIENCE OF LIFE. 



thrown into a spiral coil, and at several points it dilates, 

 to form the stomach, etc. The mouth is developed 

 from an infolding of 'skin. The liver is an outgrowth 

 from the digestive tube, at first a cluster of cells, then oi 

 follicles, and finally a true gland. The lungs first ap- 

 pear as minute buds from the upper part of the aliment- 

 ary canal, or pharynx. 



15. The transformation of the cells of the blastoderm 

 into various animal tissues is effected in various ways. 



a. An interstitial deposit of formed material may occur 

 in the bioplasm, or cell. Thus oil-globules, pigment, or 

 vacuities may greatly modify the appearance and actions 

 of the cell. The action of tannin, or boracic acid, etc., 

 upon the red blood disks of animals, shows each of them 

 to be really double, having a continuous interstitial sub- 

 stance deposited in each disk. Prof. Brucke, who first 

 investigated this structure, called the parts of the disk 

 respectively, the zooid and the cecoid, the former being 

 the part which, in the living state, contains also the 

 haemoglobulin, or red coloring matter. 



b. Cells are sometimes found scattered through an 

 intercellular material, the product of cells or of cells 

 transformed and fused together. This intercellular mass 

 may either remain continuous, or split up into fibers. 

 In this way fibrous connective tissue, cartilage, etc., may 

 be formed. (Fig. 21.) 



c. The cells which cover surfaces, and through which 

 all interchange between the body and the external world 

 is carried on, are called epithelial. They differ in shape, 

 either from mutual pressure or function, some being 

 flat and squamous, (or scaly,) and others columnar. 



