TISSUES AND ORGANS 63 



29. It would seem that during countless 

 ages evolution has slowly built up a great 

 variety of animals. This evolutionary process 

 has also affected more or less every tissue. 

 We may detect primitive types of tissue, such 

 as we find in the embryo, in the tissue of the 

 placenta and umbilical cord, in the cells 

 between the bodies of the vertebrae, in the 

 vitreous humour of the eye, in the so-called 

 lymphoid tissue found in various organs, 

 and in connective tissues generally. Cartilage 

 or gristle has preceded bone. Non-striated 

 may be regarded as more primitive than 

 striated muscle. Even some striated muscles 

 seem to be more primitive than others. These 

 muscles are usually pale, but certain muscles, 

 such as the semi-membranous muscle in a 

 rabbit's leg, are red. Red muscles contract 

 more slowly than pale muscles, and their 

 structure seems to be of a lower type. Nervous 

 tissues have passed through many forms, until 

 we reach the highly complicated cells of the 

 nerve centres. The nervous elements of the 

 sense organs also show differentiation as we 

 pass from lower to higher forms. Thus the 



