MUSCLE. 87 



Histogenesis of Heart Muscle. 



In early embryos (e. y., pigs' embryos 10-12 mm. long) the 

 heart muscle is made up of small spindle-shaped cells lying 

 close together. In cross-section they are round and contain an 

 oval nucleus. The cell protoplasm contains a more or less 

 regular network (Fig. 56). No fibril bundles are present. 



Fir;. .">>. FIG. 57. 



Cross-section of heart muscle cells of a pig's Cross-section of heart muscle cells of a pig's 



embryo 12 mm. long. (MacCallum.) embryo 25 mm. long. (MacCallum.) 



In somewhat older embryos (25 mm.) the cells are still spin- 

 dle-shaped. Around their periphery is seen a row of dark 

 masses, which are the cross-sections of newly formed fibril 

 bundles (Fig. 57). These are merely an accumulation of the 

 substance of the primitive network to form longitudinal fibril 

 bundles. In embryos 40 mm. long fibril bundles are present 

 not only around the periphery, but are also scattered here and 

 there in the more central parts of the cell. In pigs 70 mm. in 

 length the cells are no longer spindle-shaped, and have all the 

 characteristics of adult cells. 



It is apparent, then, that the continuous network spoken of 

 in the adult fibre made up of the fibril bundles and the mem- 

 branes bounding the disks of sarcoplasm, is developed directly 

 by a process of differentiation from the primitive protoplasmic 

 network of the embryonic cell. The gradual acquirement of 

 special powers of contractility is due to the progressive devel- 

 opment of the network of contractile substance present in the 

 beginning. 



