BULLETIN OF THE 
28 
partial penetration of the mesoderm on each side into the space between 
these two layers (Plate IV. Fig. 30, ms’drm.). At a later stage the 
mesoderm may be seen as a single cell layer lying between the ectoderm 
and the inner layer of the bud midway between the oral and anal ends 
(Plate IV. Fig. 32, ms’drm.), and as a double cell layer at the oral end of 
the bud (Fig. 34, ms’drm.). It is from these cells at the oral end of the 
bud that the retractor muscles are to arise (Plate III. Figs. 23-25, cl. 
mu. ret.). As the oral end of the kamptoderm and csophagus to which 
their inner ends are attached moves away from the ectoderm, and as the 
area of the latter itself increases, the two ends of the cells move farther 
and farther apart, and the young muscle cells become drawn out into 
spindle-shaped muscle fibres. (Plate III. Fig. 25, cl. mu. ret.; Plate IV. 
Fig. 36, mu. ret.) The retractor thus arises unpaired and remains so 
at its origin, but nearer its insertion in the ring canal and osophagus 
one can distinguish a division into right and left masses. The adult 
muscle fibres consist of two parts at least, the inner contractile portion 
and an outer less modified protoplasmic portion, which can be traced 
over the whole of the first part, but is most evident around the nucleus, 
where it has a granular appearance. 
b. Pyramidalis. — At about the stage of Figure 25 (Plate IIT.) one 
finds, on cross sections of the branch which pass through thé neck of the 
polypide, that the mesoderm of the body wall on each side of the neck 
is greatly thickened, and that its closely packed cells, which lie three 
or four deep, have become somewhat elongated. Cell division is quite 
common in the ectoderm of this region, and by it the area of the circum- 
cervical region is increased and the two ends of the muscle fibres are 
carried farther apart, one end remaining attached to the neck of the 
polypide and the other moving towards the abatrial surface. I have 
given reasons above (page 16) for believing that the abatrial ends of the 
muscles are not carried towards the abatrial side passively, and solely by 
the growth of the body wall, but that the ends move relatively to the 
cells of the body wall. A somewhat late stage in the development of 
the pyramidalis is shown in Figure 63 (Plate VI.). Nearly the whole 
of the mesoderm of the body wall has here been transformed into 
muscle cells. The insertion of the muscles is in the mesoderm of the 
neck of the polypide. (Plate VI. Fig. 63; Plate V. Fig. 45.) 
c. Parietal muscles first make their appearance at about the stage 
of the terminal individual of Plate II. Figure 14, immediately below 
the bud and to the right and left, i. e. so that the muscles, which 
usually arise paired, have their long axes parallel to the sagittal plane 
