MUSEUM OF COMPARATIVE ZOOLOGY. 11 



While the cuticula of Figure 18 ia seen to be that of the normal body 

 wall in this region, that shown in Figures 15 and 16 appears under the 

 microscope after staining in hsematoxylin to be of two distinct kinds : 

 (1) that outside of the central region, which is highly refractive and not 

 at all stained ; and (2) that which lies immediately over the central elon- 

 gated cells of the bud, which is also highly refractive but stains deeply. 

 In fact, the central cuticula resembles in every way that already described 

 for the tip of the branch, and shown in Plate I. Fig. 6. Moreover, it 

 has other points of resemblance to the latter. It does not stain at all 

 in alum cochineal ; the outer boundary of the branch is often uneven 

 at this place (Fig. 16) ; and particles of dirt are often found adhering to 

 it, while the rest of the cuticula is comparatively free. The difference in 

 staining properties of the central and lateral cuticulas indicates that the 

 former undergoes with age a change in its chemical properties ; the irreg- 

 ular outer boundary and the adhesion of dirt particles seem to indicate 

 that the newly formed cuticula is viscid. The mesoderm of the stage of 

 Figure 15 consists of a single loose layer of subspherical cells of the two 

 kinds already noticed, reticulated and non-reticulated. The series of 

 Figures 18, 15, and 16 shows the behavior of columnar cells in the forma- 

 tion of a typical outfolding as distinguished from the slipping in of cells 

 to form the polypide (Figs. 3, 4, and 5). 



In stages later than that of Figure 16, the tip of the branch becomes 

 further removed from the body wall of the median branch. The cells at 

 the tip always retain their elongated columnar condition. A polypide 

 is soon formed on the upper part of the body wall immediately behind 

 the tip, exactly as in the case of the median branch. A septum is 

 very early formed, cutting off the lateral from the median individual, 

 and the lateral secondary branch becomes the median primary one of 

 new individuals (Plate VI. Fig. 58). 



"VVe have already traced out the origin of the polypide of the median 

 branch from the mass of cuboidal cells near the tip ; it remains to de- 

 termine whether the cells which give rise to the lateral branch can be 

 traced directly back to the cuboidal cells of the tip, or whether they have 

 arisen from the flattened epithelium of the genei-al body wall and sec- 

 ondarily acquired their plump " embryonic " character. 



Figure 18 ( Plate II.), to the cellular conditions of which I have already 

 referred, shows an early stage of the lateral branch, and Figure 20, gm. I., 

 shows on a smaller scale the different cellular conditions in the body wall 

 in the region of two lateral buds which are yet far from showing ex- 

 ternal signs of evagination. The cells are cuboid and much higher than 



