MUSEUM OF COMPARATIVE ZOÖLOGY. 3D 
at which muscle fibres were arising. A similar series through a slightly 
older bud gives for the same regions respectively 5, 14, and 13 cells per 
section. In series through older buds, a rapid decline in the number of 
these cells occurs so that at the stage of Figure 30 (Plate IV.) there is an 
average of only about 3.1 cells per section through the bud, and about 
2.2 immediately below. These reticulated cells are not very numerous 
in the region of the bud at the time this is about to arise, as a look at 
the sections Figures 3 and 4 shows. One finds reticulated cells in the 
mesoderm at the tip, and most abundantly at a rather early stage in 
the development of the bud. The number of these cells diminishes as 
one leaves the young individual to pass into the next older of the same 
branch. In the adult such cells are rather rare; so rare, in fact, that 
Kraepelin (’87), who studied with care the body wall of the adult indi- 
vidual, makes no mention of them. Nevertheless they do occur in the 
cells which are to go into the lateral branch (Plate II. Fig. 15), as well 
as elsewhere on the body wall. The place in which one finds the reticu- 
lated cells most abundant, however, is in the young lateral branches near 
the time when the polypide bud is about to arise. Here every cell of the 
mesoderm is greatly enlarged, and filled with the vacuoles (Plate VI. 
Fig. 58). These are very apparent upon a surface view of the branches. 
Reticulated cells occur not only in the mesodermic cells of the body 
wall, but also in those of the polypide bud, which were, indeed, only 
lately a part of the mural mesoderm (Plate III. Fig. 28, Plate VI. Fig. 
56). Thus, in general terms, we may say that the reticulated cells of 
the mesoderm are chiefly confined to regions in which there are young 
buds developing; and since these arise at intervals only, there is a 
periodicity in their appearance,—a time of maximum development 
followed by one of decline, then one of reproduction of such cells in 
the ends of branches culminating in another maximum, and so on. 
Turning our attention now more particularly to the structure of these 
reticulated cells at the period of their best development, we find (Plate 
VI. Figs. 56, 57, 59) that they possess a large nucleus lying at the 
deep end of the cell and containing a relatively large nucleolus, and that 
this is surrounded by a granular protoplasm with included vacuoles. It 
is very common to find the nuclei in various stages of division, and thus 
it is frequently seen as a mass of chromatic substance without any nu- 
clear membrane or nucleochylema. The vacuoles, which in the more reg- 
ular cells lie in a semicircle nearly peripheral (the nucleus being at the 
centre), are highly variable in number, some of the cells containing as 
many as 20 to 30. They often appear as perfectly clear homogeneous 
