MUSEUM OF COMPARATIVE ZOOLOGY. 5 



exactly as the region a to S represented the tip of the branch when the 

 older polypide, whose neck is shown at cev.pyd., was of the age that the 

 younger bud is now. It will be necessary first of all to study carefully 

 each of these three regions before treating of their origin and fate. 



The tip of the branch consists of the two layers of cells which are found 

 in other parts of the body wall, — the ectoderm and the mesoderm, as the 

 coelomic epithelium may, for brevity's sake, be called. The cells of the 

 ectoderm at the extreme tip (Plate I. Fig. 6) are greatly elongated, form- 

 ing a columnar epithelium. There are about 25 or 30 of the larger cells. 

 They have a length of 28 /x to 32 p., and a diameter of about 4/x. They 

 possess an ovoid nucleus averaging 5.7 /x by 2.6 /x, which lies in the middle 

 of the cell but slightly nearer the coelomic epithelium than the cuticula. 

 It possesses a large nucleolus over 1 /* in diameter, which often appears 

 stellate owing to the threads of plasma surrounding and proceeding from 

 it and forming a nuclear network. As the figure shows, the plasma of 

 the cell is filled with large, apparently deeply stained granules, some of 

 the largest being over 0.6 /x in diameter. The coarser granules lie chiefly 

 in the immediate vicinity of the nucleus, but are also found arranged in 

 long lines at right angles to the surface throughout the greater part of the 

 cell, becoming finer the farther they lie from the nucleus. A fine network 

 can sometimes be made out between the large granules, but this appear- 

 ance is more evident at the peripheral portion of the cell, where there are 

 no large granules. At the outer and inner ends of the cells one finds large 

 vacuoles, the largest of which are of about the same size as the nucleus ; 

 these become smaller the nearer they lie to the nucleus. In many cases 

 the larger vacuoles are each seen to be partly filled by a body which stains 

 slightly, and, as focusing determines, is more highly refractive than the 

 plasma. Similar highly refi'acting, slightly staining granules are found 

 in, and in fact often composing, the smaller " vacuoles." Owing to the 

 fact that the deeply staining granules lie near the nuclei, and that the 

 vacuolated and finely granular plasma lies more remote, there is a very 

 marked deeply staining band occupying the middle of the ectodermal 

 layer, and having about four tenths the thickness of the whole layer. 



At the outer ends of the cells, and doubtless secreted by them, there is 

 a cuticula about 1 yx thick. Its inner surface is sharply marked off from 

 the underlying plasma ; its outer surface is less sharp, and there are 

 usually very minute particles of dirt attached to it (not represented in the 

 figure). The whole cuticula forms in section a continuous band of 

 substance, which stains deeply in Ehrlich's haematoxylin (but not at all 

 in alum cochineal), and covers nearly the whole tip. Looked at from 



