616 
In the frog also the cells are spindle-shaped, and definite striated 
fibril bundles are found surrounded by small sarcoplasmic discs. 
It may be stated in passing that longitudinal sections of ordinary 
voluntary muscle in the dog show a similar structure, as regards the 
relation of sarcoplasm and muscle fibril. There is in voluntary muscle, 
however, relatively less sarcoplasm, and the sarcoplasmic discs are 
much less easy to make out. 
Histogenesis of the Heart Muscle Cell. 
For the following study of the development of the heart muscle 
cell a series of pig-embryos ranging from 10 mm to 100 mm in length 
was employed. A brief description of the cells found in two or three 
of these stages will serve to illustrate the whole course of the de- 
velopment. 
The muscle cells in the heart of a pig 10 mm long vary con- 
siderably in structure according to the position which they occupy in 
the organ. At the periphery of the ventricular wall one finds cells 
which in cross section present a single row of darkly staining masses 
around the outside, surrounded by small circular areas of unstained 
substance, while the rest of the cell is made up of larger clear areas. 
In the deeper layers of the ventricular wall, there are cells which 
show no dark masses in cross section, but are made up entirely of 
a more or less regular network, the meshes of which resemble closely 
the outlines of the sarcoplasmic discs seen in adult muscle. The 
meshes of this network are in some cases broken up into smaller 
discs by a radial division. Often at the same level or generally a 
little nearer the inner surface of the ventricular wall, there occur 
cells whose cytoplasm consists of an irregular network containing in 
its meshes a clear unstained substance (Fig. 6). 
In longitudinal section one finds the cells all spindle-shaped, with 
one nucleus, situated in the centre. In some there is an irregular 
network inside the cell, and in others the meshes of this appear as 
a regular succession of discs, piled on one another in more or less 
regular rows (Fig. 7). In other cells, fibril bundles are seen at the 
periphery, and a great many of these show striations as in adult 
muscle, although in some, the fibrils seem as yet to be devoid of a 
Querscheibe. 
There are thus four distinct types of muscle cells to be found 
in a heart at this stage of development. 
I. Cells with an irregular network whose meshes contain a clear 
unstained substance (Fig. 6). 
