428 , F. H. Krecker, 



the ectoderm. As mentioned before the ectoderm in the septa forming 

 Zone is considerably enlarged and the cells are elongated approximately 

 parallel with the transverse axis of the body. Furthermore several 

 of these ectoderm cells may be seen in process of division, the division 

 occurring in such a way that one of the two resulting cells is given off 

 on the side toward the coelora. From these small cells, which thus lie 

 next to the coelom, is formed a great part of the longitudinal muscles. 

 An examiiiatioii of the figui'es will show the points at which the various 

 changes occur. Near tlie posterior end of the ectodermal thickening 

 the cells are merely enlarged (Fig. 13 and 14). Farther toward the 

 anterior end they begin to divide, in almost every case by amitosis, 

 as Seen in figures 13 and 15, the latter being a cross section of the body 

 at the level at whicli the division first occurs. Figure 16 is a cross 

 section still farther anterior and shows several of the small cells; some 

 of them have begun to elongate parallel with the circumference of the 

 body, a change of shape which is still more evident in the next cross 

 section in which the muscular dement has already appeared (Fig. 17). 

 The special muscle forming ectoderm cells are most abundant along 

 the lateral portion of the body wall and are chiefly concerned in the 

 formation of nuiscle at this point, althoiigh a few are also present on 

 the dorsal and the ventral side. In addition to these special cells the 

 ordinary ectoderm cells also form muscle fibrils at their basal ends, 

 particularly those cells in the dorsal and the ventral region. This 

 (in a certain sense) double origin of the circular muscles is similar to 

 that described by Nussbaum for the Enchytraids in which »eine jede 

 Epidermiszelle in ihrem basalen gegen die Leibeshöhle gerichteten 

 Abschnitte Muskelfibrillen produziert .... Zum Teile werden sie auch, 

 besonders in späteren Regenerationsperioden, aus speziellen Ectoderm- 

 zellen gebildet, die gegen die Leibeshöhle wandern«. 



The transformation into muscular tissue is best followed in longi- 

 tudinal sections since in these the various steps can be seen in a single 

 section (Fig. 14). At a level slightly anterior to the point at which 

 the ventral nerve cord is being formed the portion of the ectoderm 

 cells nearest the coelom loses its normal appearance and becomes 

 somewhat homogeneous in character. At first it does not stain readily, 

 in hematoxylin and eosin, but it soon shows an affinity for the eosin 

 and traces of a fibrillar structure are evident; first as an extremely 

 narrow band, at times hard to see, lying just outside of the longitudinal 

 muscles. Later the band increases in width, the ectoderm cells become 

 correspondingly shorter and the ends of the ectoderm cells nearest 



