28 BULLETIN OF THE 



partial penetration of the mesoderm on each side into the space between 

 these two layers (Plate IV. Fig. 30, ms'dnn.). 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, d. 

 mu. ret.). As the oral end of the kamptoderm and oesophagus 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, miL. ret^ The retractor thus arises unpaired and remains so 

 at its origin, but nearer its insertion in the ring canal and oesophagus . 

 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. 



h. Pyramidalis. — At about the stage of Figure 25 (Plate III.) one 

 finds, on cross sections of the branch which pass through the 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 1 6) 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 



