INTRODUCTION 



some cases it may be observed that the filaments of the intranuclear 

 network are made up of fine juxtaposed particles, arranged either in a 

 single or a double row. 



All the embryonic cells are formed from the division of the ovum 

 or egg-cell, which divides first into two cells, these again into two, and 

 so on until a large number of cells (embryonic cells) are produced. 

 Eventually these resultant cells arrange themselves in the form of a 

 membrane (blastoderm) which is composed of three layers. These 

 layers are known respectively as the ectoderm or epiblast, the meso- 

 derm or mesoblast, and the entoderm or hypoblast. The ectoderm 

 gives rise to most of the epithelial tissues and the tissues of the nervous 

 system ; the entoderm to the epithelium of the alimentary canal 

 (except the mouth), and the glands in connection with it; and the. 

 mesoderin to the connective and muscular tissues. 



The histogenetical relation between the three layers of the blasto- 

 derm and the several tissues and organs of the body is exhibited in the 

 following table : 



r The epithelium of the skin or epidermis, and its appendages 



viz., the hairs, nails, sebaceous and sweat glands. 

 The epithelium of the mouth, and of the salivary and other 



glands which open into the toouth. The enamel of the 



teeth. The gustatory organs. 

 The epithelium of the nasal passages, and the cavities and glands 



which open into them. 

 The epithelium covering the front of the eye. The crystalline 



lens. The retina. 



The epithelium lining the membranous labyrinth of the ear. 

 The epithelium lining the central canal of the spinal cord and 



the fourth, third, and lateral ventricles of the brain. 

 The tissues of the nervous system. 

 (-The pituitary body. The pineal gland. 



fThe connective tissues. 



The blood and lymph corpuscles. 



The epithelial lining of the heart, blood-vessels, lymphatics, and 

 serous membranes. 



The epithelium of the uriniferous tubules (in part). 



The epithelium of the generative organs, and the generative pro- 

 ducts in both sexes. 



The muscular tissues, voluntary, involuntary and cardiac (except 

 the muscular fibres of the sweat glands, which are epiblastic 

 in origin). 

 I The spleen and other lymphatic and vascular glands. 



The epithelium of the alimentary canal (from the pharynx to 

 the lower end of the rectum) and all the glands which open 

 into it (including the liver and pancreas) . 



The epithelium of the Eustachian tube and cavity of the tym- 

 panum. 



-j The epithelium of the larynx, trachea, and bronchi, and of all 

 their ramifications. The epithelium of the pulmonary 

 alveoli. 



The epithelium of the thyroid body. Part of the thymus gland. 



The epithelium of the urinary bladder and ureters, and of part 

 of the uriniferous tubules. 



B2 



Ectoderm 



or 

 Epiblast 



Mesoderrn 



or 

 Mesoblast 



Entoderm 



or 

 Hypoblast 



