AGE, GROWTH, AND DEATH 



early state of the embryo. They represent something- 

 different and new. 



The next of our illustrations, Figs. 19 and 20, 



show us muscle fibres of the 



sort which serve for our volun- 



tarymotions 



and are 



connected 



ty p ically 



with some 



part of the 



skele t o n . 



These mus- 



clefibresare 



FIG. 19. CROSS SECTION 

 OF A LINGUAL MUSCLE FIBRE 

 OF THE MOCASSIN SNAKE, AN- elongated 

 CISTRODON PISCIVORUS. The 

 single large dark spot repre- 

 sents a nucleus. Each small 

 dot represents a cross section 

 of a muscle fibril. There are 

 several hundred in each fibre. 



structures. 



Each fibre 



contains a 



contractile 



substance 



different from protoplasm, and 

 which exists in the form of delicate 

 fibrils which run lengthwise in the 

 muscle fibres (Fig. 19), and is so 

 disposed, further, that a series of 

 fine lines are produced across the 

 fibre itself (Fig. 20), each line cor- 

 responding with a special sort of 

 material different from the original 

 protoplasm. These cross lines give to the voluntary 



FIG. 20. PART OF A 

 MUSCLE FIBRE OF THE 

 HUMAN TONGUE TO 

 SHOW THE CROSS STRI- 

 ATIONS. Two nuclei 

 are included, one of 

 which is shown at the 

 edge of the fibre, the 

 other in surface view. 

 In the adult striated 

 muscle fibres of mam- 

 mals the nuclei are su- 

 perficially placed. 



