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are not confined to the actual periphery of the cell, but are scattered 
irregularly throughout the peripheral parts of the cytoplasm. 
At 20 mm there is a more marked change, although the four 
types of cells can still be made out. Here, however, one finds a 
greater proportion of cells containing fibril bundles, and also those 
with the large sarcoplasmic discs divided into smaller discs are now 
more numerous. 
In an embryo 26 mm long, nearly all the heart muscle cells 
contain fibril bundles. Those cells which are devoid of fibrils are of 
the types II and III described above. 
At 35 mm I can make out no cells without fibril bundles. In 
embryos 44 mm in length there are no cells without fibril bundles. 
Usually, there is a row of fibril bundles around the periphery, and 
scattered bundles also throughout the rest of the cell. In every case 
each bundle is surrounded by small sarcoplasmic discs. 
Embryos 55 mm in length present cells which are still spindle- 
shaped, although sometimes they are considerably elongated. In cross 
section there is a marked resemblance to the adult fibre, but the 
fibril bundles are all polygonal in cross section, and there are none 
of the band-like bundles such as are found in fully developed cells. 
The heart muscle cells of embryos which have attained the length 
of 72 mm are no longer spindle-shaped as they were before, but have 
almost the form of the adult muscle fibre. In cross section a great 
many fibril bundles are seen in each cell, but there are still no band- 
like bundles at the periphery. It would appear that the band-like 
bundles of adult muscle result from the further growth of the peri- 
pheral fibril bundles. The cells at this stage are much smaller in 
cross section than in adult muscle. The general appearance at this 
age and in tissue from a 100 mm pig approaches very closely to 
that of the adult muscle. 
It is an interesting fact that in all the early stages, more highly 
developed cells occur at the periphery of the heart, while in the deeper 
layers of the ventricular wall the cells are as a rule younger. The 
cells, therefore, seem to develop at the inner side of the muscle layer. 
Karyokinetic figures are very abundant throughout the muscle, but 
most numerous towards the inside of the heart. 
From this brief synopsis of the stages found in early pig embryos, 
it will be seen that there is a complete transition from cells with 
simple structure, and containing no fibrils, to the complicated structure 
found in adult muscle. The cell with irregular network, type I, must 
