THE STUDY OF CHICK EMBRYOS 



ferentiated network of cells which fill in the spaces between the definite layers 

 (epitnelia). This tissue is mesenchyma (Fig. 53). The mesoderm may be largely 

 converted into mesenchyma as in the head, or any of the mesodermal layers 

 may contribute to its formation. Thus it may be derived from the primitive 

 segments and from the somatic and splanchnic mesoderm. The cells of the 

 mesenchyma form a syncytium or network, and are at first packed closely to- 

 gether. Later, they may form a more open network with cytoplasmic processes 

 extending from cell to cell (Fig. 53). The mesenchyma is an important tissue of 

 the embryo, as from it are differentiated the blood and lymphatic systems, 



together with most of the smooth muscle, 

 connective tissue, and skeletal tissue of 

 the body. 



The body of the embryo is now com- 

 posed (i) of cells arranged in layers epi- 

 thelia, and (2) of diffuse mesenchyma. The 

 term "epithelium" may be used in a 

 general sense or restricted to layers cover- 

 ing the surface of the body or lining the 

 digestive canal and its derivatives. Layers 

 lining the body cavities are termed meso- 

 thelia, while those lining the blood-vessels 

 and heart are called endothelia. 



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

 the epithelia and mesenchyma of the three germ layers as follows: 



Ectoderm 



Mesen- 

 chyma 



FIG. 53. Mesenchyma from the head of a 

 thirty-six-hour chick embryo. X 495. 



Ectoderm 



1. Epidermis and its deriva- 

 tives (hair, nails, glands) . 



2. Conjunctiva and lens of eye. 



3. Sensory epithelia of organs 

 of special sense. 



4. Epithelium of mouth, enam- 

 el of teeth, oral glands. 

 Pituitary body. 



5. Epithelium of anus. 



6. Epithelium of amnion and 

 chorion. 



7. Nervous, neuroglia and 

 chromamn cells of nervous 

 system. Retina and optic 

 nerve. 



8. Notochord (?) 



9. 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 uro-genital or- 

 gans. 



6. Striated muscle. 



1. Skeletal. 



2. Cardiac. 



B. Mesenchyma. 



1. Blood-cells. 



2. Bone marrow. 



3. Endothelium of blood-vessels. 



4. Endothelium of lymphatics and 

 spleen. 



5. Supporting tissues. (Connect- 

 ing tissue, cartilage and bone.) 



6. Smooth muscle. 



Entoderm 



1. Epithelium of digestive 

 tract. 



2. Liver. 



3. Pancreas. 



4. Epithelium of pharynx. 



Eustachian tube. 

 Tonsils. 

 Thymus. 

 Thyreoids. 

 Epithelial bodies. 



5. Epithelium of respiratory 

 tract. 



Larynx. 

 Trachea. 

 Lungs. 



6. Notochord (?) 



