BIOLOGICAL INTRODUCTION 5 



isms confirms the evidence of the past history, or phylogeny. Each 

 multicellular organism develops from a single fertilized egg, or oosperm. 



The fertilized cell at first divides, and divides again to form many 

 similar cells. These cells then become differentiated into various 

 tissues, which eventually become grouped to form different organs 

 (p. 773). In the course of development a three-layer blastoderm 

 is formed. 



From the outer layer ectoderm chiefly the epithelial protective 

 and nervous tissues are developed; from the middle layer mesoderm 

 the supporting and muscular tissues; from the inner endoderm 

 the respiratory and alimentary tissues. 



By the name of epithelium is designated the tissue lining the 

 outer and inner surfaces of the body. Occupying this position, its 

 main function is that of transferring material from the surfaces to 

 the tissues within. It also plays a part in protecting the underlying 

 organs, and in receiving stimuli and transforming these into sensory 

 nerve impulses. From it, also, the glandular structures of the body 

 are developed e.g., salivary glands, glands of stomach, intestine, etc. 



The supporting and connective tissues are developed to give 

 rigidity and tensile strength to protoplasm, and thus enable the mul- 

 ticellular organism to preserve a definite form. For these purposes 

 fibres, plates, and such massive structures as the bones, are formed. 

 The ligamentum nuchse of the ox affords an excellent example of 

 tensile strength, while the shells of molluscs and cartilaginous and 

 bony structures testify to the great rigidity which may be developed. 

 Cells of like origin may also act as a storehouse of food material 

 e.g., fat. 



The muscular tissue has become developed, not only to move the 

 organism from place to place, but also to assist in the internal opera- 

 tions of the stationary organism for example, the heart, cilia, etc. 

 Its essential property is the power of contraction. 



By a combination of tliesa three tissues, epithelial, connective, 

 and muscular, the outer covaring parts, or integument, of the body is 

 formed. One of the chief functions of this integumsnt is to protect 

 the exposed outer parts of the body from dangers to which it is sub- 

 jected, whether the animal live in the water or in the air. This it 

 can do both mechanically and by the production of means of defence 

 e.g., poison. The integument possesses also lubricating and cleans- 

 ing powers. Sometimes, too, it has the power to produce attractive 

 or repulsive odours, to prepare adhesive material, or to spin. 



Examples of mechanical protection are seen in the stiff fibres 

 developed by many lower animals e.g., the turbellarian worms, to 

 protect against undue pressure; the cuticle of the earthworm; the 

 carapace of the lobster and the shell of the tortoise; the scales of 

 fishes and of birds' legs; the feathers of birds and the hair of animals; 

 the outer covering of the human skin generally, and in particular the 

 thickened areas of the palms of the hands and soles of the feet. 



Various examples of offensive protection may also be given. Formed 

 within the cell are the trichocyst of paramoscium, the rhabdites or 



