•360 
\VIL[;IAM F. ALL EX. 
[Figs. 19-25 have been introduced to show the comparative rate of 
growth and histogenesis of the mjotomes and the muscnli cordis 
candalis.] 
Fig. 19. — Ti-ansverse section through the lower portion of the 
^embryonic myotoines and the embryonic muscnli cordis candalis of a 
20 mm. Polistotrema, series A. In this stage the myoblasts in 
the ventral end of the myotoines are not as far advanced as they are 
further dorsad, having no sign of fibrillse, and no sign of perimysium 
has thus far appeared. It is continuous, however, with a greatly 
thickened mass of mesenchyme, which has probably migrated inward 
to form the connective tissue for the caudal heart and other structures, 
and in this greatly thickened mass of mesenchyme there may be 
muscnli cordis candalis myoblasts (?) indistinguishable from the 
adjacent mesenchyme, and which may have migrated in from the meso- 
blastic somites with the mesenchyme. X 130. 
Fig. 20. — From an identical transection to Fig. 19, but from a 27 mm. 
embryo, showing the same structures slightly advanced. Note the 
formation of the external jierimysium and the separation of the myo- 
tonies from the thickened mass of mesenchyme described above, and 
tliat the protoplasmic processes of this mesenchyme has undergone a 
change possibly preparatory to the forming of fibrous connective-tissue 
fibres, while the m. cordis candalis boundaries are outlined. X 130. 
Fig. 21. — (1) On the left, a transection of a small portion of the 
embryonic myotonies taken from about the centre. (2) On the right, 
similar portion of the muscnli cordis candalis in transection. Note 
that the fibrillae and internal perimysium have made their appearance 
in the myotoines, but not in the m. cordis candalis. X 225. 
Fig. 22. — Identical to Fig. 21, but from a 27 mm. series. External 
perimysium has appeared in the myotonies and a slight change has 
•occurred in the intercellular spaces of the embryonic in. cordis candalis. 
X 225. 
Fig. 23. — Identical to Fig. 21, but from a 60 mm. embryo. The 
muscle-fibres of the myotonies are fairly well developed, and fibrillse 
are making their first appearance in the myoblasts of the in. caudis 
eaudalis. X 225. 
Fig. 2-4. — Identical to Fig. 21, but from an 85 mm. embryo. The 
muscle-fibres of both the myotomes and the in. cordis candalis are well- 
formed. Note the presence of Cohnheim's areas in the former and the 
much larger size of the fibres in the myotomes. X 225. 
Fig. 25. — Identical to fig. 21, but from a small adult 10 cm. long. 
Botli fibres have attained adult conditions. Note how much smaller 
the 111 . cordis candalis fibre is than the myotome fibre. The ni. cordis 
■candalis is also extremely rich in blood-vessels, x 225. 
