778 DIRECTIONS FOE LABORATORY WORK 



(4) Sketch (92) portion of developing myocardium from a stained 

 transverse section of some mammalian embryo (e.g., pig embryo of from 

 10 to 20 mm. length). Note the shape, and the myofibrillar cytoplasmic 

 content, of the constituent myoblasts. Do the myoblasts anastomose? 

 Is heart muscle originally syncytial in character? 



(C) STRIPED (Striated; Voluntary striped}. 



(1) Tease a fragment of fresh frog's or other vertebrate's muscle in 

 normal salt solution, or in Ringer's solution. Mount under cover-glass 

 and study carefully, noting position of nuclei, longitudinal myofibrils, 

 and cross striations (J or isotropic discs; Q or anisotropic discs, and 

 ground membranes or membranes of Krause). By exerting pressure on 

 cover-glass, crush some of the fibers. Look for the sarcolemma spanning 

 a break in the sarcoplasm. (This is especially readily demonstrated in 

 the frog's muscle.) Add drop of dilute acetic acid and note effect. Then 

 add a drop of the methylene blue stain, observe the shape and position 

 of the nuclei, and sketch (93) a fiber, including the sarcolemma at the 

 level of fracture. 



(2) Study fibers from a macerated preparation, preserved in the 

 alcohol-glycerin mixture. Note the terminal cleavage of some of the 

 fibers into discoid structures, the sarcomeres ; and the splitting of others 

 into the constituent myofibrillae (sarcostyles). Sketch (94) a fiber 

 showing cleavage into sarcomeres and (95) one showing splitting into 

 sarcostyles. 



(3) Study a stained transverse section of striped muscle (e.g., 

 tongue; skeletal muscle). Draw (96) a number of adjacent fibers, indi- 

 cating nuclei, myofibrils, Cohnheim's areas (Kolliker's columns), and 

 sarcolemma. Sketch (97) a fiber in longitudinal section, showing the 

 several cross striations. How do the cross striations in striped voluntary 

 muscle differ from those in cardiac muscle? 



(4) Make drawing (98) of a portion of a longitudinal section of 

 some arthropod striped muscle fiber (e.g., insect leg or wing muscle) 

 from a demonstration preparation, showing several phases in the con- 

 traction process. Note the several striations: isotropic and aniostropic 

 discs, Z and M membranes (telophragmata and mesophragmata) ; and 

 the additional stripes of: (a) the accessory disc (of von Eollet and 

 Englemann) and (b) the resulting terminal disc (of Merkel). Explain 

 different arrangement of the striations in contracted and extended fibers. 

 Difference between a contraction 'band' and a contraction 'wave'? 



(5) Sketch (99) l.p. a portion of a transverse section of a small 



