54 



THE STUDY OF CHICK EMBRYOS 



from the somatic and splanchnic mesoderm. The cells of the mesenchyme form 

 a syncytium or network, and are at first packed closely together. Later, they 

 may form a more open network with cytoplasmic processes extending from cell 

 to cell (Fig. 55). The mesenchym gjs^an importan t tissu e of the embryo; from 

 it are differentiated the blood and b 



_ together with most o f the 

 smooth muscle, connective tissue, and s kele- 

 tal tissue_o f the body . 



The body of the embryo is now com- 

 posed (1) of cells arranged in layers — epi- 

 thelia, and (2) of diffuse mesenchyme. The 

 term "epithehum" may be used in a general 

 sense, or restricted to layers covering the 

 surface of the body or Hning the digestive 

 canal and its derivatives. Layers hning the 

 body cavities are termed mesothelia^ while 

 those Hning the blood vessels and heart are 

 called ^ndotkdia. 



Derivatives of the Germ Layers. — The tissues of the adult are derived from 

 the epitheUa and mesenchyme of the three germ layers as follows: 



Ectoderm 



Mesenchyme- 



Fig. 55. — Mesenchyme from the head of a 

 thirty-eight-hour chick embryo. X 495. 



Ectoderm 



1. Epidermis and its derivatives 



(hair, nails, glands). 



2. Conjunctiva and lens of eye. 



3. Sensory epithelia of organs of 



special sense. 



4. Epithelium of mouth, enamel 



of teeth, oral glands. Hypo- 

 physis. 



5. EpitheUum of anus. 



6. Male urethra (distad). 



7. EpitheUum of amnion and 



chorion. 



8. Nervous, neurogha, and chro- 



maffin cells of nervous sys- 

 tem. Retina and optic nerve. 



9. Notochord (?). 



10. Smooth muscle of sweat glands 

 and of iris. 



Mesoderm 



A. Mesothelium. 



1. Pericardium. 



2. Pleura. 



3. Peritoneum. 



4. Serous layer of intestine. 



5. Epithelium .of most of urogeni- 



tal organs. 



6. Striated muscle. 



1. Skeletal. 



2. Cardiac. 



B. Mesenchyme. 



1. Blood ceUs. 



2. Bone marrow. 



3. Endothelium of blood vessels. 



4. Endothelium of lymphatics. 



5. Spleen and lymphoid organs. 



6. Supporting tissues. (Connect- 



ing tissue, cartilage, and bone.) 



7. Smooth muscle. 



Entoderm 



1. EpitheUum of digestive 



tract. 



2. Liver. 



3. Pancreas. 



4. EpitheUum of pharynx. 



Eustachian tube. 

 Tonsils. 

 ThjTnus. 

 Thyreoids. 

 Para thyreoids. 



5. EpitheUum of respiratory 



tract. 

 Larmx. 

 Trachea. 

 Lungs. 



6. EpitheUum of most of blad- 



der, of female urethra, 

 male prostatic urethra 

 and prostate. 



7. Notochord (?). 



For the histological development (histogenesis) of the various tissues from 

 the primary germ layers see Chapter X. 



