128 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



active segment, and is especially well marked in its posterior half (Plate 

 3, Fig. 1G). This cell multiplication may continue in certain places 

 until the layer en'drm. 2 becomes five or six cells deep (Figs. 7-9). 

 The cytoplasm of the older layer (eri'drm. 1 ) stains feebly and diffusely, 

 while that of the new layer (en'drm. 2 ) takes a very deep stain. The 

 new entodermal layer attains a greater thickness in the posterior than 

 in the anterior zooid. The dorsal portion of the digestive tube of the 

 posterior zooid, beginning just behind the brain and extending to the 

 posterior limit of the budding zone, is the region of greatest thickening. 

 The thickness diminishes gradually from the dorsal to the ventral side 

 of the tube. Later a separation of the old from the new entoderm 

 occurs, and a cavity — crescent-shaped in cross section, the lumen of 

 the new pharynx — appears {phy. lu., Figs. 9, 1G). The new ento- 

 dermal cells next take on the typical columnar form and then become 

 ciliated (Plate 3, Fig. 17 ; Plate 4, Fig. 19). The action of the cilia is 

 manifest before the separation of the zooids, and before the rudimentary 

 lumen of the pharynx has any connection with either the old lumen or 

 the outside world. The old entodermal lining finally becomes detached 

 throughout the pharyngeal tract (Plate 2, Fig. 14, erfdrm}) and is swal- 

 lowed or ejected through the mouth. 



It is important to note that the entodermal cells are even more com- 

 pletely separated into two functionally different regions, corresponding 

 to the two zooids, than are the cells of the ectoderm. There is a distinct 

 neutral zone between them, embracing the plane of division. This in- 

 terruption is most conspicuous dorsally and laterally, being only slight 

 ventrally (Plate 3, Fig. 16). 



In the anterior zooid, immediately in front of the future plane of sep- 

 aration, two pairs of thickenings arise in a manner entirely analogous 

 to that just described for the pharynx of the posterior zooid (Plate 3, 

 Figs. 16, 17; Plate 4, Fig. 23; Plate 5, Figs. 24, 25, pav.), and in 

 these thickenings spaces are formed, as in the case of the pharynx. Ulti- 

 mately the dorsal and ventral cavities on the same side coalesce, and 

 likewise the right and left communicate across the median plane, both 

 dorsally and ventrally, thus forming a complete annular space of varying 

 calibre around the old tube. These new entodermal cells, like those of 

 the pharyngeal wall, bear cilia ; they constitute the lining of the pavilion. 

 The ectoderm has no part in forming the inner face of the pavilion, the 

 concrescence of the free margins of the two layers, ectoderm and ento- 

 derm, taking place at the margin of the pavilion. The digitiform 

 process, however, is wholly covered with ectoderm, the boundary be- 



