MUSEUM OF COMPARATIVE ZOÖLOGY. 5 
exactly as the region a to § represented the Zip 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 
coolomic 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 p to 32 a, and a diameter of about 4u. They 
possess an ovoid nucleus averaging 5.7 a by 2.6 a, which lies in the middle 
of the cell but slightly nearer the coolomic epithelium than the cuticula. 
It possesses a large nucleolus over la in diameter, which often appears 
stellate owing to the threads of plasma surrounding and proceeding from 
it and forminga 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 u 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 lic 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 refracting, slightly staining granules are found 
Owing to the 
in, and in fact often composing, the smaller ‘ vacuoles.” 
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 thick, Its inner surface is sharply marked off from 
the underlying plasma; its onter 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 hematoxylin (but not at all 
in alum cochineal), and covers nearly the whole tip. Looked at from 
