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WILLIAM F. ALLEN. 
immediate region of this muscle ; while the preceding 
ventral spinal nerve (Fig. 19, V. iS'p. N.) has not reached a 
level nearly as far vent rad. 
Inthe 27 mm. series (Fig. 20) several important changes have 
occurred in the region of the future m. cordis candalis. The 
former reticular appearance of the mesenchyme has to a large 
extent disappeared, and in place of it, bunches of embryonic 
white fibres have appeared everywhere about and between 
the cells; the ventral boundaries of the myotomes {viyo.} 
have descended considerably ventrad, and in place of one 
solid mass of concentrated mesenchyme plus a few hypo- 
thetical undifferentiated myoblasts, as was described for the 
20 mm. series as lying between the myotomes and the caudal 
heart, we have in the 27 mm. series (Fig. 20), and more 
strongly marked out in sections further cephalad, two quite 
distinct areas. First a dense area situated directly laterad of 
the caudal heart is composed of a great many round or 
slightly spindle-shaped cells separated by a small amount of 
embryonic white fibres, and a second thinner area, located 
between the first area and the myotomes, consists of but few 
cells and more embryonic fibres. The first area will un- 
doubtedly form the lateral connective-tissue layer of the 
caudal heart and the m. cordis candalis; although in this 
series the myoblasts cannot be detected from the embryonic 
connective-tissue cells, still the general shape of the area 
itself in transverse section resembles that of the m. cordis 
candalis of a later stage. The second or thinner area repre- 
sents nothing more than mesenchyme changing into white 
fibrous tissue. 
Between the 27 mm. and the 58 or 60 mm. series there is a 
considerable gap, resulting naturally in a marked progress in 
the histogenesis of the two areas described in the previous 
paragraph. The first area has differentiated into the lateral 
connective-tissue wall of the caudal heart (Fig. 16, F. C. T.) 
and into the m. cordis candalis (Fig. 16, m.c.c.) ; while the 
second area is now a mass of embryonic white fibrous tissue, 
connecting the m. cordis candalis with th.e myotomes. A 
