6 BULLETESr OF THE 



the surface after staining in hsematoxylin, it appears uniformly dark. 

 The mesoderm of the tip is highly modified, and a description of it 

 will be more instructive after I shall have described the normal coelomic 

 epithelium, as I shall do later. 



Passing from the extreme tip towards fi (Fig. 14), one finds the ecto- 

 dermal cells gradually changing in form, size, and structure, and becoming 

 slightly broader, and very much shorter. Their nuclei lie near the inner 

 ends of the cells, possess a thick ''nuclear membrane," and are more 

 nearly spherical than those of the columnar cells, but of about the same 

 size. They each possess one very large, centrally placed nucleolus, whose 

 diameter equals and sometimes exceeds one third that of the nucleus, and 

 whose outline is often somewhat stellate. Outside of the nucleus in the 

 cell body there are fewer and fewer vacuoles as we pass from the tip, but 

 the plasma is still coarsely granular, and here, as before, these stained 

 granules surround the nucleus. It is now the regions between cells 

 rather than those at the inner and outer ends which remain unstained, 

 so that the cells are sepai-ated from one another by light spaces. 



The mesodermal layer becomes somewhat thinner than at the tip, that 

 is to say, its cells are flattened. The nuclei are elongated in the axis of 

 the branch, and average about 4 /x by 2.2 /x. They possess one spheri- 

 cal nucleolus, whose diameter is about two thirds of the minor axis of 

 the nucleus. Small, clear vacuoles often with highly refractive spherical 

 bodies are abundant in the cell protoplasm, which stains as a whole less 

 deeply than does the ectoderm. Such highly vacuolated elements will 

 be called reticulated cells. 



If we study the gemmiparons zone at a stage considerably earlier than 

 that shown in Figure 14, in fact at a stage in which a polypide is about to 

 arise, we find an appearance of the layers represented by Plate I. Fig. 3. 

 In such a region the ectoderm consists of cuboid cells about 7 n high by 

 6.5 ft broad. The nuclei are large, nearly spherical, and vary in size from 

 3.5 /x to 6.0 fi. The largest nuclei are those in the region from which a bud 

 is about to arise (ex.'). One in this region (to the right o? ex.) is 6.5 /x 

 by 6.0 /x in diameter, with a nearly spherical, eccentrically placed nucleo- 

 lus of about 3.0 /i in diameter. This nucleus is the largest which I have 

 found in the whole tissue of Paludicella, and the same is true of the nucle- 

 olus. From the examination of many regions from which buds are about 

 to arise, I can assert that such regions always, in Paludicella, possess large 

 nuclei and large deeply staining nucleoli. / I shall have occasion to de- 

 scribe similar conditions elsewhere, and to point out the probable signifi- 

 cance of these facts. The cell body possesses a highly granular, deeply 



