DEVELOPMENT OF DISCS, HEART MUSCLE OF PIG 335 
He found that there were four different types of cells in the heart 
of the 10-mm. pig. One presented a network of irregular meshes 
in which there was a clear, unstained substance; another, a 
regular network; a third, a network in which some of the meshes 
were broken up into smaller discs by radial division, and 
fourth, a network in which a single row of fibrils began to ap- 
pear. The later stages of development showed an increased 
number of fibril bundles. The cells of 55-mm. embryo hearts 
were still spindle-shaped, but very much lengthened, and in the 
72-mm. and 100-mm. stages the cells had lost the spindle form 
and had taken almost the form of the adult muscle. The fibrils 
were found to be all through the cell rather than just at the 
periphery. MacCallum described the adult muscle of the 
human heart as being composed of rhomboidal cells two or three 
times wider than long. They sometimes broke up into branches 
which united with branches from other cells. The lines of union 
were at acute angles to the length of the fibers and each fibril 
of the cells sent out two or three processes which ran through 
these lines of union and met each other. He represented by his 
figure 1 a very complex structure and stated that the same 
structure was not to be found in other animals, but that the 
structures commonly known as protoplasmic bridges corre- 
sponded to his lines of union in the human heart. The cells 
were composed of fibrils which were stained dark and sur- 
rounded by unstained sarcoplasm. He described the fibrils with 
their surrounding sarcoplasm as made up of dises separated from 
each other by a narrow line known as Krause’s membrane. He 
found the sarcoplasmic discs also to be divided somewhat radi- 
ally and the lines of separation were continuous with Krause’s 
membrane. 
In the embryonic tissue MacCallum noticed that the cells at 
the periphery of the heart were farther developed than those at 
the anterior, indicating that the cells grow on the inside of the 
muscle. He considered that the development of the cells from 
an irregular network to cells of the adult form with numerous 
fibril bundles increased the heart’s capacity for work and that 
the network of the earlier stages must be contractile. 
