158 THE LATER STAGES OF DEVELOPMENT 



of the organs, and modify their growth and histological differ- 

 entiation, or the arrangement of their cells and tissues. We 

 shall now briefly review this 'Afunctional stage'' of development. 

 The best known phenomenon in this context is the hypertrophy 

 of an organ caused by excessive use of that organ. If one kidney 

 of a vertebrate animal is extirpated, the other kidney increases 

 in size. Another well known example is the great increase in 

 the size of striated muscle as a consequence of heavy muscular 

 work. 



The influence of the functional relationships on the arrange- 

 ment of tissue elements is clearly illustrated by Weiss's (1929- 

 33) experiments. He exposed tissue cultures of connective tissue 

 cells (fibroblasts) to local tension. The cells then became 

 arranged in accordance with the direction of the lines of tension, 

 and multiplied more rapidly in this direction. In normal devel- 

 opment, such reactions play a role in the origin and regenerat'on 

 of tendons, etc., as was shown by Lewy's experiments as early 

 as 1904. The detailed architecture of bones obeys the same law. 

 The trabeculae (bone bars) in spongy bone are arranged in the 

 direction of the stresses normally acting on the bone. If the 

 direction of these forces is modified, e.g. by the extraction of 

 a molar from the jaw, the direction of the trabeculae is changed 

 accordingly. The growth of bones also depends upon their 

 function. A motionless limb which is not subjected to any stress 

 is retarded in growth, as compared with a normally functioning 

 limb. 



Function is of paramount importance also for the final 

 structure of the vascular system. With the exception of the 

 very first blood vessels, which differentiate locally, all others 

 arise in the course of embryonic development by the branching 

 of existing vessels. Initially, massive "buds" of endothelium cells 

 are formed. These extend into the tissues and soon become 

 hollow. Manifold connections between the outgrowing blood 

 vessels are then formed, so that a network develops. At first, 

 all these vessels are of about the same calibre. The same 

 process takes place in regeneration. This primitive network 

 differentiates into the definitive vascular system in the follow- 

 ing way. Some of its meshes disappear, others become capil- 



