196 The Fibrils of the Heart Muscle Cell of the Chick 
or below the nucleus. Some of the meshes of the cytoplasmic network 
are divided into smaller parts, the small sarcoplasmic discs of MacCallum, 
97 (Fig. 6, sd). 
120-130-hours’ stage—In Figs. 8 and 9 are. shown characteristic 
longitudinal sections of the cell at this stage in its development. The 
eyto-reticulum has undergone a striking change. Instead of the irregular 
structure of the preceding stages, we find a definite arrangement of the 
network into rectangular meshes. The deposits on the reticulum have 
increased in bulk in such a way that in longitudinal section they appear ~ 
as oval-shaped bodies (Fig. 9,a). In Fig. 9, which represents a slight 
advance in the development of the cell over that shown in Fig. 8, inter- 
posed between every two adjacent longitudinal threads of the network 
bearing the deposits, is to be seen a longitudinal thread the intersections 
of which with the transverse threads are not marked by heavily staining 
deposits (Fig. 9, ba). 
This re-arrangement of the meshwork is apparently the first step in 
the laying down of the fibril bundles. Subsequent development shows 
that the longitudinal strands bearing the deposits represent the axes 
of the fibril bundles, and the deposits, the Querscheibe of the adult tissue. 
The transverse threads (Fig. 9,¢) according to MacCallum, 98, give rise 
(in the mammal) to Krause’s membrane. However, this will be referred 
to again later. 
Cross sections (Fig. 10) show the meshes of the reticulum to be 
further subdivided, but few of the original size remaining. It is to be 
noted that one or two very large, irregularly shaped meshes are present 
in this section (Fig. 10, Ja). Apparently these areas, in later stages, 
become divided into smaller parts, just as the neighboring cytoplasm has 
already become divided. Evidently this is the manner in which the 
erowth process in the cell takes place, the cytoplasm in the smaller 
meshes increasing greatly in bulk and then, by subdivision, producing 
a number of meshes approximately the size of the first. 
The nucleus is oblong in longitudinal section (Fig. 8,d; Fig. 9, d), 
and roughly circular in cross section (Fig. 10, d). The chromatin 
masses are small, although Fig. 10 shows one of considerable. size. 
It appears that Eycleshymer, 04, in his work on skeletal muscle-cells 
of Necturus, found just the opposite change to take place in the size 
and distribution of the chromatin granules, 7. e., in younger stages the 
karyosomes were evenly distributed in the nucleus, and in later stages 
collected in large masses. 
130-140-hours’ stage——The most interesting phase of the entire devel- 
