286 



HISTOGENESIS 

 A 



Mesenchymal cell 



Fihrilla in ectoplasmic 

 matrix 



I Mesenchymal cell 



j Cartilage matrix 



Cell of syncytium 

 Elastic fiber 



Cartilage cell 

 Fig. 291.— The differentiation of the supporting tissues (after Mall). X 270. A, White fibers 

 forming in the corium 6f a 5 cm. pig embryo; B, elastic fibers forming in the syncytium of the umbilical 

 cord from a 7 cm. embryo; C, developing cartilage from the occipital bone of a 20 mm. pig embryo. 



into typical white fibers. The spindle-shaped cells are transformed into the 



connective tissue cells characteristic of the adult. In tendons, the bundles of 



^ white fibers are arranged in compact parallel fascicles, in 



areolar tissue they are interwoven to form a meshwork. 



The cells of the tendons are compressed between the 



bundles of fibers and this accounts for their peculiar form 



and arrangement. In the cornea of the eye the ceUs retain 



^^ip \ their processes. The corneal tissue is thus embryonic 



^5w -=-.!-». in character and is without elastic fibers or blood vessels. 



Elastic Tissue. — With the exception of the cornea 

 and tendon, yellow elastic fibers develop in connection 

 with all white fibrous connective tissue. Like the 

 white fibers they are produced in the ectoplasm of the 

 mesenchymal syncytium (Fig. 291 B). They are de- 

 veloped as single fibers, but may coalesce to form the 

 fenestrated membranes of the arteries. According to 

 Ranvier, elastic fibers are produced by the union of ecto- 

 plasmic granules, but this view is not supported by either Mall or Spalteholz. 



Fig. 292.— Develop- 

 ing fat cells, the fat black- 

 ened with osmic acid (after 

 Ranvier). n., Nucleus; g., 

 fat globules. 



