THE SUPPORTING TISSUES 



293 



divided into two stages: (i) a prefibrous stage during which the ectoplasm is 

 formed rapidly by the endoplasm of the cells, and fibrils resembling those of 

 reticular tissue appear in the ectoplasm (Fig. 283, A}. (2) The anastomosing 

 fibers take the form of parallel bundles and are converted through a chemical 

 change 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 be- 



Mesenchymal 

 cell 



Fibrillae in 



ectoplasm c 



matrix 



f 



Ceil of 

 Jyncyf/urn 



Elastic 

 fiber \ 



B 



c 



MesenchymaJ Cell 

 I Carfilaqe matrix 



< 



Sb> 



\\\ & M 



*V" . * x 



Cartilage cell 



FIG. 283. Figures showing the differentiation of the supporting tissues (after Mall). A , white fibers 

 forming in the dermis of 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. 



tween the bundles of fibers and this accounts for their peculiar form and arrange- 

 ment. In the cornea of the eye the cells retain their processes. The corneal 

 tissue is thus embryonic 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. 283, B}. They are developed as single fibers, but may coalesce to form the 

 fenestrated membranes of the arteries. According to Ranvier, elastic fibers are 



