128 



THE TISSUES 



Fibrillar arrangement of the spongioplasm first appears around the periphery 

 where granules form in the cytoplasm and become arranged in rows lengthwise 

 of the cell. The central portion of the cell is at this stage still occupied by retic- 

 ular spongioplasm and the nucleus. These granules next unite to form deli- 

 cate fibrils. New fibrils form both by longitudinal arrangement of more granules 

 and by longitudinal splitting of fibrils already formed, until they finally fill 

 the entire cell. After the union of the granules to form fibrils the latter are 

 apparently homogeneous, but later differentiate into the alternate light and 

 dark substances which determine the cross striations characteristic of striated 

 muscle. Just how this differentiation takes place is not known. During this 

 process of fibrillation the nucleus has been undergoing mitotic division without 

 corresponding division of the cytoplasm. In white fibres these nuclei migrate 

 to the surface and come to lie just beneath the sarcolemma. The sarcoplasm in 

 which the fibrils lie probably represents the remains of the undifferentiated pro- 

 toplasm (hyaloplasm). 



i«iS=5^t^'**l-'-,'>"'':v-=-==?^^-i=1?^^§^?^^ 



Fig. 73. — From a Section of Developing Heart ^luscle in a Rabbit Embrj-o of 10 mm. 



(Godlewski.) 

 The fibrils are segmented, indicating the beginning of the cros5 striation characteris- 

 tic of heart muscle. 



Some authorities deny the origin of the muscle fibre from a single cell, describ- 

 ing it as derived from a number of myoblasts which unite to form a fibre. 



Heart muscle develops from mesenchyme. The myoblasts are at first small, 

 spheroidal, and closely arranged. With the appearance of intercellular substance 

 the cells become separated and irregular in shape, and anastomose to form a syn- 

 cytium. A little later the cells become arranged in parallel columns and cross 

 markings, the " intercallated discs" appear, dividing the syncytium into the 

 so-called "heart-muscle cells." Whether these coincide with the original myo- 

 blasts is not known. As in voluntary muscle, fibrils first develop in the periphery 

 of the cell apparently by union of granules which arrange themselves in lines 

 lengthwise of the cell. The fibrils increase in number and invade the entire 

 cell except the small area of undifferentiated protoplasm which remains around 

 the nucleus. Later the fibrils show the alternate light and dark markings or cross 

 striations. As with voluntary muscle the way in which these cross markings 

 develop is not known. 



Attention has already been called (p. 5,0 to the spongioplasm as the con- 

 tractile element of protoplasm. It is to be noted that in the development of 



