202 The Fibrils of the Heart Muscle Cell of the Chick 
In the case of the chick, however, the above is not true. For if we 
consider the longitudinal threads of the network of the stage represented 
in Figs. 8 and 9, as the incipient fibril bundles, then the deposits mark- 
ing the intersections of the threads would represent the striations, since 
later these develop into the Querscheibe of the adult tissue. The question 
is just what is to be understood by “ first appearance of fibrille.” If 
by this we mean the earliest stage in the development of the fibril bundles 
at which there is any resemblance to the adult structure, we may say 
that the fibrillee are striated from the start in the chick. 
With reference to another point in this connection, the writer here 
quotes a passage from the same author, pp. 298, 299: “A point of 
capital importance is found in the fact that in Necturus, Amia Lepi- 
dosteus . . . . as my own observations show, and in other forms 
as Kaestner, 92, has found, the beginning of fibrillation is coincident 
with the first contractions. The movements of the embryo first begin 
in the anterior of the mid-dorsal myotomes and in these the myoblasts 
are first fibrillated. The above considerations led the writer to support 
the theory that the fibrille are pre-existent structures and represent the 
principal contractile element.” 
The same argument cannot be applied to the heart muscle of the chick, 
because the first contractions occur at the time when about 15-17 myo- 
tomes have been formed in the embryo. As may be seen from the 
representation of the heart muscle cell at this stage (Fig. 1), no 
structure which might be truly called a fibril is present. The first 
appearance of anything that in the faintest way resembles the adult 
is not seen before the 120-130-hours’ stage (Figs. 8 and 9). 
If the fibrille are not pre-existent structures, what then are the 
contractile elements in the very early embryonic stages? In view of the 
above facts the explanation offered by MacCallum, 97 (p. 620), seems 
plausible. This author suggests that the contractile elements in the 
early embryonic heart would be represented by the irregular network 
seen at that stage before true fibrils exist. This, it seems, would lend 
support to the writer’s suggestion in regard to the origin of the fibrils. 
For if, as MacCallum says, the cytoplasmic network represents the con- 
tractile element in the early stages, it seems reasonable at least to con- 
sider the longitudinal lines of this network as developing into the fibrils 
rather than to suppose the latter to arise from accumulations of the 
network-substance in the cytoplasm contained between the meshes. 
In conclusion, the results of the work embodied in this paper point 
to the existence of a definite relationship between the cytoplasmic reti- 
culum of the early embryonic cell and the fibril bundles of the adult cell. 
