198 The Fibrils of the Heart Muscle Cell of the Chick 
Cross sections (Figs. 13 and 14) show an increased number of the 
smaller meshes of the cyto-reticulum, while the deeply staining deposits 
(a) which are cross sections of fibril bundles have become greatly en- 
larged. These heavily staining patches are best developed toward the 
periphery of the cell. The nucleus is roughly oval in outline. 
From this structure to that of the adult is but a step. While all the 
intermediate stages between this and the adult were not examined, 
sections from stages beyond this up to the eighth day, showed the same 
structure in various degrees of development. In the adult the sarco- 
plasmic discs are better developed. The Querscheibe are broader and 
resemble thick, flat plates. The length and diameter of the cell is greatly 
increased. 
SUMMARY. 
The facts which appear to be of importance as set forth in the fore- 
going, are as follows: 
1. The cytoplasm of the early embryonic heart cell is traversed by an 
irregular network, the nodes of which are marked by heavily staining 
deposits. 
2. This network tends to become more and more regular, until its 
strands are longitudinally and transversely disposed. 
3. The heavily staining deposits on the primitive network develop into 
the Querscheibe of the adult fibril bundle. 
4. The longitudinally disposed lines of, the network represent the axes 
of the fibril bundles of the adult. 
5. The sarcoplasmic discs of the adult develop from the inter-reticular 
cytoplasm of the embryonic cell. 
GENERAL DISCUSSION ON THE RELATION BETWEEN THE CYTOPLASM AND 
THE FIBRIL BUNDLES. 
According to Ranvier, 89, the myocardium of the mammal is com- 
posed of rhomboidal branching cells. The nucleus is centrally placed 
and is surrounded by a granular mass stretching out in the axis of the 
cell. Surrounding this granular mass is the contractile element which 
shows longitudinal and transverse markings. This element is the fibril 
and is made up of successive segments having the same structure as 
voluntary muscle. 
Koelliker, 02, describes the mammalian heart muscle as composed of 
an interlacing network of cells having centrally placed nuclei. The 
contractile substance, as in voluntary muscle, consists of fibril bundles, 
the so-called “ Muskelsaéulchen,” which show a definite transverse striping. 
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