178 
pheral fibre-aspect of these cells. The one structure found to occupy 
this position was the cellular and fibrillar investing perimysium of 
the muscle fibre. Hence such muscle cells lie outside of the sarco- 
lemma. The latter envelope encloses only the highly specialized 
muscle fibrillae with the semi-fluid sacroplasm. Therefore, for these 
additional reasons the muscle fibrillae are to be regarded in the 
adult as extra- or intercellular structures. 
An analogy between the histogenetic cycle of the connective 
tissue group and that of voluntary striped muscle can be drawn. 
The extremes of the cycle of the latter have been established by a 
number of competent observers. The intermediate steps, however, 
require further study. At first the myofibrillae are laid down in the 
genetic cell bodies. At the opposite end of the genetic course these 
fibrillae are extracellular. Parallel is the course of development in 
the connective tissue group; at first appearing as intracellular fibrillae 
and later being extruded from these genetic cell bodies. These facts 
obtain as well in the instance of cardiac musculature of the adult 
white mouse. 
In the study of this form of striped muscle fibre the same 
technic was employed as that detailed in the cited papers. The sec- 
tions varied in thickness from 2 u to 31/, u; the stain used was 
alcoholic hematoxylin. 
The first figure represents a longitudinal section of several 
muscle fibres of the ventricle. The marked morphological structural 
differences between the protoplasma immediately investing the nuclei 
and the sarcoplasm of the fibre are apparent at first observation. The 
cellular spongio-plasmatic network of the former is wanting in the 
latter. These two forms of protoplasm do not blend with each other; 
rather, they are sharply delimited from each other by a distinct mem- 
brane, the cell wall. With the alcoholic hematoxylin, this structure 
stained deeply in contrast to the neighbouring parallel-running muscle 
fibrillae. In focusing down through the section the uninterrupted, 
membrane-like nature of the cell wall could be readily noted. In 
contrast to this fact the slender muscle fibrillae pass into and out of 
focus as their level was reached and passed. Again, the features of 
cross-striation were not observed on the cell wall, hence the identity 
of the membrane apart from the muscle fibrillae was established. 
The question of the longitudinal extent of such cells, as was the 
case with similar sections of voluntary striped muscle, is still un- 
