galloway: non-sexual reproduction in dero vaga. 123 



muscular wall (Plate 1, Fig. 2, z' ; Plate 2, Figs. 11, 12). The thickening 

 is first manifest on the ventral aspect, extending ultimately as a girdle 

 around the segment. It is interseptal in position, and about equally 

 distant from the nearest dissepiments. That this thickening is a matter 

 of growth and not of local contraction is shown both by the increased 

 number of cells and the increased length of the segment. The peripheral 

 margins of the dissepiments bounding the bud-zone are crowded apart, 

 as^ indicated in Figure 2. The bristle bundles belonging to the involved 

 segment are forced backward into contact with the following dissepi- 

 ment. They become the bundles of the 5th setigerous segment of the 

 posterior zooid. An external groove (Plate 1, Figs. 4, 5, sul.) is next 

 formed in the ectodermal thickening, and persists as the outward demar- 

 cation between the zooids, indicating the place of ultimate separation. 

 This groove deepens gradually but unequally at different points of its 

 circumference. The activity of the tissue is strictly confined to one seg- 

 ment, the segments in front of and behind this one undergoing no appre- 

 ciable changes. Hence, it is possible to affirm that the tissues of the 

 active segment posterior to the plane of the groove furnish the material 

 out of which are formed the prostomium, four new segments, and the 

 anterior portion of the fifth segment, with their contained structures. 

 Similarly, from the anterior half of the segment spring the whole set of 

 structures appropriate to the tail end of the worm. It is possible to 

 locate the fundaments of these organs from the beginning of the process. 



The division of ectodermal cells may continue until a thickness of 

 three or four cells is reached in the dermal layer. In the mean time, as 

 may be seen in transverse sections, the proliferating ectodermal tissue 

 breaks through the muscular layer into the body cavity, especially in 

 the spaces between the longitudinal muscle bands (Figs. 8-10). These 

 invading elements on either side the median plane coalesce with each 

 other and with proliferations of the cells from the ventral nerve cord, 

 filling the ventral and lateral portions of the body cavity in a very char- 

 acteristic way (Plate 1, Figs. 6, 7). In a sagittal section, a differentia- 

 tion of this internal cell-mass into an anterior and posterior portion is 

 apparent at a relatively early stage (Fig. 5). The cells on the con- 

 tiguous faces of these masses become arranged into bounding surfaces, 

 which are ultimately continuous with the external layer, and become, in 

 part, the boundaries of the deepening groove. Thus the latter is supple- 

 mented in its deeper portion by an internal delamination, and the two 

 together produce deep invaginations, especially from the latero-ventral 

 regions (Plate 3, Figs. 16, 17, sul), which are paired and contribute to 



