344 
WILLIAM F. ALLFX. 
in the next section ceplialnd a temporary opening is present, 
but on the right or opposite side to Pig. 17. Just how tlie 
permanent valves are constructed out of this rather coarse 
sieve-like valve-fold^ or Avhy these valve-folds should fuse at 
all, my material does not demonstrate, but would expect the 
permanent opening in the centre to occur by the gradual dis- 
integration of the tissue in that region. Transverse sections 
apparently sliow that this valve-fold extends dorsad into the 
heart, cephalad and caudad of the immediate regions of the 
anterior and posterior ventral orifices. 
A still later stage, as shown in the 8o mm. series, demon- 
strates considerable change in the valves guardingthe Amntral 
orifices of the heart both as regards form and general 
structure. Instead of appearing short and broad in section, 
as in Fig. 16, they have become long and slender bands of 
Avell-differentiated connective tissue, lined Avith endothelium. 
As was observed in the 60 mm. series the second or posterior 
valves are considerably less advanced than the first or anterior 
pair. In the 85 mm. series the posterior pair are in about the 
same stage as the anterior pair in the 60 mm. series. 
VII. Histogenesis oe the Musculi Cordis Caudalis. 
In discussing the origin of this muscle it Avill be necessary 
to liegin Avith the myotonies, and throughout a comparison of 
the two is of interest on account of their close relationship 
both as regards position and innervation. 
If a comparison be made of the dorso-Amntral extent of the 
myotomes in the 20 mm. series (Fig. 5, myo.) and an 85 mm. 
stage (Fig. 1, Myo.) in Avhich adult conditions are practically 
reached, it will be seen that the embryonic myotomes in the 
20 mm. series must grow considerabl}^, both dorsad and 
ventrad, before their relative adult dimensions are reached. 
A higher magnification reveals the central part of the 
myotome in the 20 mm. series to be much further advanced 
than is either the dorsal or the Amntral end. Here the 
myoblasts (Fig. 21, Myo. Myoh.) shoAv the beginnings of 
