619 
be considered as the earliest stage inasmuch as it occurs most abund- 
antly in the youngest embryos, and is not present at all in older 
ones. 
The cells which contain fibril bundles are certainly older than 
the other stages, for the fibril bundles are characteristic of adult 
muscle, and occur only in small numbers in early embryos. In chick 
embryos at the third day no fibrils at all are present, although the 
heart beats quite vigorously. The other two stages, types II and III, 
fall between these two extremes, and that in which there is a break- 
ing up of the regular network into smaller discs, is older than that 
containing the large simple discs, as it approaches more closely to 
the adult structure. 
There is thus a definite developmental sequence formed by the 
following five stages: 
1) Cells with a simple irregular network. 
2) Cells with a regular network consisting of large sarcoplasmic 
discs. 
3) Cells in which the large sarcoplasmic discs have been broken 
up with the formation of the small sarcoplasmic discs. 
4) Cells in which fibril bundles have formed at the junction of 
the small sarcoplasmic discs with one another. 
5) Adult cells as described above. 
Considered in the third dimension the first stage consists of an 
irregular network filling up the cell. This, on further differentiation, 
becomes regular resulting in stage 2. This consists of regular discs 
piled upon one another in rows running in the long axis of the cell. 
The primitive network is still present, but has become more regular, 
and forms the boundaries of the large sarcoplasmic discs. Stage 3 is 
arrived at by the breaking up of the large sarcoplasmic discs, by a 
radial division into small sarcoplasmic discs. At the nodal points, or 
at the junction of two or more small sarcoplasmic discs, the fibril 
bundles develop, and stage 4 is reached. This may, apparently, take 
place between any two or more small sarcoplasmic discs, but more 
especially does it occur at the central point of the breaking up of a 
large sarcoplasmic disc. After this stage, the development is a process 
of repetition and growth, giving rise finally to the adult structure. 
The possibility of a close connection, or even identity, of the sub- 
stances making up the fibril bundles, and the substances which form 
the stainable network gives opportunity for pleasing hypothesis. If 
this relation be proven to exist, the partition separating the small 
